MANUAL OF AGRICULTURE. 241 



be entire, serrated, crenate, and so on ; the blades, according to 

 tlie apex, acute, obtuse, &c. When divided laterally from mar- 

 gin to midrib they cleft pinnately ; longitudinally, they are pal- 

 mately cleft, and so forth. 



Vernation is the varied mode of the folding up of young leaves 

 in the bud. The attachment of leaves to stem is spirally arranged 

 in a strict mathematical order. There is the like analogy be- 

 tween branches and stems, rootlets and root. Certain plants 

 lose their leaves annually, others retain them, permanently. Of 

 the first division, the leaves of some wither and fall away on the 

 completion of bud formation. Sucli plants are called deciduous, 

 as oak and ash trees. Of others, the leaves wither and decay, 

 but still adhere, as do those of lilies. Plants of the second divi- 

 sion retain their leaves of one season's growth till the full 

 development of their successors in the next ; the majority of our 

 evergreens for example. When their functions have been nearly 

 discharged, leaves chanc^^e their colour, and from the secretion of 

 inorganic matter in their cells, they shrivel up. Simultaneously, 

 a constriction of the base of the petiole becomes gradually com- 

 plete, whereupon all the cells of stalk and leaf die, and the latter 

 falls to the ground. 



Leaf functions are analogous to those of the lungs. Leaves 

 seem to expose plant sap to the action of air and light, which 

 frees their juices from excessive moisture, and induces such 

 chemical chancres of their substance as elaborate them into suit- 

 able compounds for assimilation by the plant, to the end that in 

 all its parts cell-building may multiply. 



In certain plants the leaves are possessed of strange supple- 

 mentary powers. For instance, the leaf of Venus's fly-trap has 

 the property of curling inwards and enfolding the luckless 

 insect which may have alighted thereon. And stranger still, 

 this duress is effected from a carnivorous propensity ; for physio- 

 logists declare that the plant thereupon absorbs the juices of the 

 insect for its own nourishment. In the pitcher plant some of the 

 leaves act as watertight reservoirs, by assuming the form and 

 direction best suited for receiving the supply of moisture ; and 

 frequently they contain a considerable supply of water. The 

 large quantity of fluid containing solid and gaseous bodies in 

 solution, and absorbed by the delicate cells of roots, lias an 

 upward current through the central portion of the stem, and 

 reaching the leaves, where it undergoes the changes adverted to, 

 it next takes a downward current through the interior parts of 

 the stem, delivering growth materials through its course. 



The belief at one time was general of the excretory power of 

 roots over plant waste and matter injurious to its health ; but it 

 has been surrendered by contemporary physiologists. The absorb- 

 ent and exhalcnt power of plants over moisture has, it will be seen, 



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