LESSON 25.] ANATOMY OF THE BOOT. 149 



LESSON XXV. 



ANATOMY OF THE EOOT, STEM, AND LEAVES. 



416. Having in the last preceding Lessons learned what the 

 materials of the vegetable fabric are, we may now briefly consider 

 how they are put together, and how they act in carrying on the 

 plant's operations. 



417. The root and the stem are so much alike in their internal 

 structure, that a description of the anatomy of the latter will answer 

 for the former also. 



418. The Struetare of the Rootlets, however, or the tip of the root, 



demands a moment's attention. The tip of the root is the newest 

 part, and is constantly renewing itself so long as the plant is active 

 (67). It is shown magnified in Fig. 56, and is the same in all rootlets 

 as in the first root of the seedling. The new roots, or their new 

 parts, are mainly concerned in imbibing moisture from the ground ; . 

 and the newer they are, the more actively do they absorb. The ab- 

 sorbing ends of roots are entirely composed of soft, new, and very 

 thin-walled cellular tissue ; it is only farther back that some wood- 

 cells and ducts are found. The moisture (and probably also air) 

 presented to them is absorbed through the delicate walls, which, like 

 those of the cells in the interior, are destitute of openings or pores 

 visible even under the highest possible magnifying power. 



419. But as the rootlet grows older, the cells of its external layer 

 harden their waUs, and form a sort of skin, or epidermis (like that 

 which everywhere covers the stem and foliage above ground), which 

 greatly checks absorption. Hoots accordingly cease very actively to 

 imbibe moisture almost as soon as they stop growing (67). 



420. Many of the cells of the surface of young rootlets send out a 

 prolongation in the form of a slender hair-like tube, closed of course 

 at the apex, but at the base opening into the cavity of the cell. 

 These tubes or root-hairs (shown in Fig. 55 and 56, and a few of 

 them, more magnified, in Fig. 337 and 338), sent out in all direc- 

 tions into the soil, vastly increase the amount of absorbing surface 

 which the root presents to it. 



421. Structure of the Stem (also of the body of the root). At the 

 beginning, when the root and stem spring from the seed, they consist 



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