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annual heat is not found beyond the 27th degree of latitude ;
consequently the plantain and sugar cane will not ripen in the
open air in a higher latitude; and this Baron Humboldt has
found to correspond with the height of 3000 feet under the
equator. The cotton plant will not flourish without 68 degrees
of heat, which is not found beyond 34 degrees of latitude,
which corresponds with about 3600 feet of elevation at the
equator. The same reasoning applies to all other plants, with
the exceptions arising from warm vallies, moisture of air, and
richness of soil.
See Plate I.

Feet.
The highest spot on which man ever trod,

19,400
The highest limit of the lichen plant,

18,225 The lowest limit of perpetual snow under the equator, 15,730 The highest limit of pines under the equator,

12,801 The highest limit of trees under the equator,

11,125 The highest limit of oaks under the equator,

10,500 The highest limit of the Peruvian bark tree,

9,500 The lowest limit of pines under the equator,

5,685 The highest limit of palms and bananas,

3,280

LECTURE XLII.

Plants as affected by cultivation, frc.

You may recollect that we have before remarked upon the permanence of species, and have observed that although they may in some respects be varied by cultivation, yet their distinctive characters will not be wholly lost. The differences which exist in species are expressed by the terms races, varieties, and variations.

Races are those differences in a species which are of a striking kind, and continued from the parent plant to its offspring, by being propagated by the seed. They are produced by strewing pollen of one species upon the pistils of another; the seed thus formed will produce a plant resembling both.

Varieties are a less important distinction than races ; they are not continued by means of the seed, but produced by grafting or continuation of the plant under some new circumstances.

Variations denote the slightest kinds of difference; they are occasioned by peculiarities of climate, soil, moisture, dry.

Elevation produces similar effects on vegetation, as distance from the equator-Permanence of species--Races—Varieties—Variations.

ness &c.

All these terms, races, varieties and variations, are often used indiscriminately for each other.

Degeneration or change of the organs of plants. The organs of plants, owing to peculiar causes, often experi. ence a metamorphosis, and instead of their usual appearance exhibit anomalies of vegetable deformities.

We use here the term deformity, as signifying any variation from the ordinary course of nature. The causes which produce these changes are,

1st. The adhesion of parts usually separate ; thus we often see flowers, leaves and fruits united, and appearing double.

Some writers, among whom is the celebrated French bota. nist, De Candolle, assert that the single petal which forms the corolla of many flowers, as the stramonium or the blue bell, is in reality composed of several petals which become soldered, or cohere together before the flower expands. The same writers consider a monophyllous calyx, to be composed of sev. eral little leaves thus united before their developement.

2d. Changes are occasioned by a want of vigour in the plant to bring all the parts to maturity. Some of the seeds thus often fail for want of nourishment; many plants which in one flower produce several seeds, often ripen no more than

The horse-chesnut has six seeds, but seldom matures more than two; in the blossom of the oak where six seeds are produced, but one acorn is perfected.

3d. In some cases organs appear from certain changes to be incapable of performing their original offices, and thus exhibit deformities; as where a bud is formed, which for want of suffi. cient nourishment, or some other cause, does not develope itself into a leaf, but forms a permanent protuberance or swelling upon the stem. The prickly pear exhibits a thick and ex. panded stem, which is formed of leaves imperfectly developed.

4th. The stamens and pistils through excess of nourishment, swell out, and become petals ; all double flowers are formed in this manner. The poppy in its natural state has many stamens, and but four petals; but you often see double poppies, with scarcely the vestige of a stamen left; the same change may be observed in the rose, which naturally has but five petals and many stamens and pistils, but in a very full, double rose, scarcely any appearance of either stamen or pistil is to be seen.

The stamens more frequently than the pistils meet

one.

Changes of the organs of plants-Deformities—1st cause for the changes of plants-2d cause of change-3d case in which changes appear-4th. Double flowers, how caused.

with this metamorphosis ; as they appear to be more intimately connected with the petals than the pistils.

5th. The petioles or foot stalks often change to leaves. This may be seen in an Arabian plant ACACIA zilotica, which furnishes the gum arabic. This tree at first exhibits upon one petiole six or eight pair of leaves; this number every year becomes less, until all the leaves disappear; the petiole then retaining all the nourishment which before was distributed to the leaves, flattens and expands, and appears in the form of a thick leaf. The trees which we call Acacia are not of this genus, but of the genus Robinia.

6th. The peduncles and petioles, sometimes change into tendrils, as in the vine ; this plant at first throws out many large leaves and clusters of flowers; but the food not being sufficient to support such a profuse vegetation, the new leaves and clusters of flowers appear smaller; the nourishment be. coming still more scanty, at length neither flower or leaf is developed, and the peduncle and petiole become tendrils, which by attaching themselves to some firm bodies, serve to sustain the rich fruit which is perfected on the flower parts of the branch,

7th. The last change we shall notice is the transformation of buds into thorns. When a plant forms more buds than it can nourish, some of them do not develope branches and leaves, but becoming hardened by the accumulation of sap, which is insufficient for their full perfection, they exhibit the short indurated process called a thorn. It is said that wild plants by rich cultivation, do in time become divested of their thorns, which change into what they seemed originally destined for, viz. leaves and branches.

Prickles, such as may be seen upon the rose, gooseberry and other plants, do not change by cultivation, for these are a natural appendage, originating from the bark; while the thorn may be found connected with the wood, of which it seems to

make a part.

Diseases of Plants.* The diseases of plants, for these organized beings are, like animals, subject to disease and death, may in many cases arise from causes within the knowledge of the attentive naturalist.

Ist. We notice constitutional diseases. Of this class are

This constitutes a department of Botany called pathology; a term derived from two Greek words, pathos, disease, and logos, account.

5th. Changes in petioles—6th. Peduncles and petioles become vines—7th. Buds, how transformed--Prickles-Diseases of plants—Ist. Constitutional.

the varied colours of some leaves, such as the box and holly; this is supposed to be owing to certain juices which by changing their elements, vary the colour of the leaf.

2d. Plants seem diseased by being subjected to too great or too scanty a supply of food, as light, heat, water, air and soil. Excess of light causes an escape of oxygen, and a too rapid deposit of carbon; the sap, incapable of sustaining so great a degree of action, becomes exhausted, the plant withers, and the leaves fall off. In this situation the food should be either increased by watering or the vegetation retarded by diminish. ing the light. Excess of heat absorbs the juices of the plant ; deficiency of heat produces dropsy and the plant losing its leaves ultimately decomposes. More water is evaporated by a plant than is retained for its nourishment; therefore the more that is absorbed by the roots, the more should be evaporated by the leaves.

3d. External injuries often affect the health of plants. Rains injure the wood by penetrating through apertures in the bark. The bark seems from its nature better fitted to bear the action of the weather. Winds when violent are mechan. ically destructive to vegetables; when moderate, the agitation which they produce is thought to be advantageous, by favouring the descent of the cambium, and promoting a more free circulation of the other juices.

Smoke is injurious to plants, it being composed of particles, which although invisible to our sight, are yet too gross to be absorbed by the minute pores of the leaves; it serves there. fore, to obstruct the pores, and prevent their exhaling the oxygen gas which is necessary for the decomposition of the carbonic acid and the consequent deposition of carbon.

4th. Plants sustain injuries from animals which produce diseases. Insects in particular make their way into the bark and external coats of the plant and deposit their eggs; these eggs when hatched produce larvæ, which often, by their pecu. liar juices, rot the wood. These insects are called cynips. One kind produces the hard protuberances on trees of different kinds, which are called gall-nuts, or nut-galls; others which are softer and more spongy are called apple-galls or berry. galls. Another kind of insect, called cochineal, attaches itself to the bark of trees, and preys upon the juices. One species of the cochineal is of a brilliant scarlet colour and much valued for its use in dyeing; this species feeds on the Cactus opuntia, a Mexican plant.

5th. Diseases are produced by plants preying upon each

2d. Light and heat-3d. External injuries--Rains-Winds --Smoke--4th. Animals.

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other, either by fastening themselves upon their surfaces, or by so near a location as to deprive others of their necessary food. Parasites fasten themselves upon the surfaces of other plants; they are distinguished into two kinds, the false and true parasites ; the former adheres to the plant without feeding on its juices, as mosses and lichens. These derive their nourishment from the atmosphere, but they injure the tree by har. boring insects, and attracting moisture, which often rots the part of the stem on which they grow. The misletoe is a true parasite whose root, piercing the bark of trees, plants itself in the alburnum, and absorbs food from it, in the same manner as if it were fixed in the soil. The Pterospora is a very curious parasite which is sometimes found upon the leaves of shrubs,* but more frequently upon the branches and leaves of trees. Mushrooms are of the class of false parasites. Smut is a black fungus, which fastens itself upon the ears of oats and other grain. The rot is a fungus excrescence which preys upon the seed; if seeds which have this disease fastened upon them are sown, the rot will be propagated also. Ergot is a disease mostly confined to rye. Rust is chiefly confined to the grasses ; both are of the fungi family.

6th. Diseases resulting from age. Plants differ from animals in one important circumstance; the latter develope their organs at once, these organs in progress of time become indurated and obstructed, until they at length decay from old age. Plants, on the contrary, renew themselves every year; that is, they form new vessels to convey the juices, new leaves to elaborate them and new buds to produce flowers and fruits. Plants do not, then, like animals, seem destined to die with old age; or there does not seem to be in perennial plants any prescribed term of existence. The producing of fruit appears to exhaust the vital energy of the plant, in annuals in one year, in biennials in two, in perennials, in a longer or shorter period according to their natural constitution, and the quantity of fruit which they produce. Apple trees which bear heavy loads of fruit, are very short lived in comparison with the oak, which perfects from each flower, but one of six seeds, and this fruit is but a small acorn.

There are some trees now known to exist, which are supposed to be of great age ; in the Island of Teneriffe is the DRACÆNA draco, which according to many circumstances may seem to

A species of this genus was found in the woods east of Troy, upon the leaf of the Vaccinium. The colour of the whole plant, consisting of two flowers, and a kind of leaf, was that of a red rose.

5th. Parasites---6th. Diseases resulting from old age-Aged trees.

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