TREE-FORM OUT OF ITS UNIT. 17 



to the earth and atmosphere, the two sources from whence it 

 must for the future draw its supplies of food. Its little root 

 descends into the soil, and puts forth from its surface a 

 number of fine, white, hairlike fibres, which are the instru- 

 ments by means of which the plants absorb inwardly the 

 subterranean nutrient material which surrounds them ; its 

 young stem ascends into the air, and the bark and fibre, 

 which are arranged cylindrically in separate beds or layers 

 in the stem, are spread out horizontally towards its summit, 

 in the form of a flat green plate, or absorbent surface, 

 called a leaf. 



The bark, or cellular tissue of this leaf is penetrated by 

 the fibrous portion of the stem, in the shape of veins and 

 veinlets, which communicate freely with the roots in the 

 soil, and thus act as conduits of the sap or nutritive mate- 

 rial from one extremity of the plant to the other. In this 

 manner the sap, brought from all parts of the plant, be- 

 comes, as it were, thoroughly spread out and aerated in the 

 leaf. To facilitate the processes of evaporation and absorp- 

 tion, the leaf is provided with an epidermis, through the 

 pores or openings of which the superfluous water of the sap 

 is evaporated, and such gases absorbed from the atmosphere 

 as are nutritious to the plant. 



Hence, when fully developed, this leaf aerates the sap 

 much more perfectly than the nursing-leaves ; and as it is 

 a true aerial leaf, it remains permanently attached to the 

 stem or vegetable axis till the close of the vegetative sea- 

 son. This leaf appears, in fact, to be formed in reference 

 to the atmosphere. 



If we examine the gemmule, or young bud, situated just 

 above the phyton, or first leaf, we shall find that the first 

 leaf itself is now, in its turn, a nursing-leaf, and the parent 

 of a numerous progeny of already partially-formed follicles. 

 These, nourished by the sap elaborated in the first leaf, soon 

 individually expand and separate from each other, little in- 

 tervals of stem being formed between them. They now 

 contribute individually to each other's support : the lower 

 leaves aiding in the development of those that are above 



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