VEGETABLE PHYSIOLOGY. 



Leaves. Leaves, from their number, position, 

 and delicacy of organisation, are designed to effect 

 an important office in the vegetable economy. 

 Springing from the branches, and exposed in 

 profusion to the atmosphere, they perform the 

 functions of a breathing-apparatus analogous to 

 that of the lungs or gills of animals. A similar 

 purpose at least is designed ; for the sap of plants, 

 like the blood of animals, requires to be exposed 

 to the atmospheric influence, in order that it may 

 l>e suitable for nutrition. This purpose is accom- 

 plished by the agency of the leaves, to which the 

 sap, on rising from the roots through the stem and 

 branches, is propelled or attracted, and there both 

 air and light exercise their beneficial influences. 

 Leaves are thus indispensable to the growth of 

 plants, and care should be taken not to injure 

 them ; for defoliation, either naturally or by art or 

 accident, instantly arrests the growth, and the 

 failure or diminished expansion of foliage is a 

 certain sign of debility. 



A leaf consists generally of two parts the 



petiole, or leaf-stalk; and the lamina, or blade. 

 Sometimes, however, as in the rose tribe, stipules 

 (s, s) are attached to the base of the petiole. The 

 leaf-stalk (a) is that part which connects the leaf 

 with the branch, and at the base will be found 

 slightly hollowed, in which a bud rests. Some- 

 times the leaf-stalk is wanting, and thus the leaf is 

 said to be sessile. In some Australian acacias 

 and eucalypti, the petiole is flattened, and occupies 

 the place of the leaves. Such petioles are called 

 Phyllodia. The lamina, or broad part of the leaf 

 {), is frequently of a different colour on the under 

 side. This is exemplified in the common silver- 

 weed (Potentilla anserine?), the leaves of which are 

 hoary on the lower side, and green on the upper. 

 Leaves are either deciduous falling in autumn 

 or evergreen, lasting till the following season. 

 Their forms are exceedingly varied being simple 

 or compound; and these, again, are distinguished 

 as oval, lanceolate, cordate, hastate, sagittate, 

 pinnate, &c. 



An important character is afforded by the vena- 

 tion of leaves that is, the arrangement of what 

 are called their veins. In exogenous or dicotyle- 

 donous plants as our timber trees, for example 

 the leaf is furnished with a strong midrib, from 

 which secondary veins diverge, at regular dis- 

 tances ; and these, again, branch off into still 

 smaller tertiary veins, whose ramifications form a 

 reticulation or network in the leaf. In endogenous 

 or monocotyledonous plants as grasses and lilies 

 there is usually no distinct midrib, but several 



primary veins, which originate in the base of the 

 leaf, and proceed to its apex, being parallel through- 

 out. In many tropical endogens, however, there is 

 a distinct midrib, from which secondary parallel 

 veins diverge on either half of the lamina. In 

 acrogens or acotyledonous plants as ferns the 

 veins are usually arranged in a forked manner. 



The veins of leaves consist of woody tissue, 

 accompanied by spiral vessels, the interspaces 

 being filled up with the softer parenchyma, or 

 cellular tissue. By maceration, the latter may be 

 made to disappear, so as to leave merely the veins. 

 In aquatic plants, there is a strong tendency to 

 non-development of the parenchyma ; thus, in 

 many of the aquatic crow-foots, the submerged 

 leaves consist almost wholly of veins, while the 

 floating ones are entire ; but the most remarkable 

 instance of this occurs in the Ouvirandra fenes- 

 tralis, a water-plant introduced to our hot-houses 

 from Madagascar, whose leaves are so destitute of 

 parenchyma, that they are, in fact, beautiful lattice- 

 like skeleton -leaves. The leaves of the royal 

 water-lily are perforated with minute holes at 

 regular intervals, and large open spaces occur in 

 the leaves of Monstera and Dracontium. 



With regard to the manner in which leaves pro- 

 ject from the branches, and their distribution over 

 the woody cylinder to which they are attached, 

 every possible variety may be observed. They 

 may be opposite that is, two leaves growing on 

 either side of the branch, the one directly opposite 

 to the other ; alternate, when one leaf springs out 

 on one side of the branch, and another on the 

 opposite side, a little above it, and so on ; whorled, 

 or verticillate, when a number of leaves grow 

 round the stem from a common knot or joint, as 

 in the bed-straw. The distribution of alternate 

 and opposite, however, is not regular ; for in some 

 instances it will be found that the leaves on the 

 lower part of the stem are alternate, whilst those 

 on the upper are opposite. There are many 

 plants which have few or no leaves, but whose 

 stems are much dilated, presenting a large super- 

 ficies of parenchymatous exterior to the air and 

 light as, for example, in the cactuses. 



Green is the most general colour of leaves, but 

 some are red, or purple, or yellow ; some appear 

 nearly white, in consequence of being clothed with 

 short woolly or silky hair. Leaves are often varie- 

 gated, in consequence of the non-development of 

 chlorophyll in the cells of certain parts. They 

 differ much in substance and structure : some are 

 immensely thick and fleshy, as those of the genera 

 Aloe and Agave; others remarkably thin, as those 

 of the beech. 



ORGANS OF REPRODUCTION. 



The organs of reproduction are the flower, fruit, 

 and seeds ; and these, or some modification of 

 them, exist in every perfect Phanerogamous or 

 flowering-plant. 



Flower. A flower consists of several distinct 

 parts the calyx, corolla, stamens, and pistil A 

 flower is essentially constituted by the presence of 

 the last two, which are the sexual organs. When 

 there is only one of these present, the plant is 

 termed unisexual ; but more commonly these 

 organs are both present in the same flower, which 

 is in this case termed hermaphrodite. In some 

 instances, although the same plant bears both 



