28 THE STEM. 



has two leaves. It is readily known by the stem consisting of 

 regular concentric layers of wood, and the leaves having reticulated 

 veins, while the number of floral organs is usually five or a multi- 

 ple of that number. All of these characters are not always coin- 

 cident, but one or more of them serve to mark the variety. De 

 Candolle called this class of plants exogens, which means to 

 grow outwardly, which is true with regard to the wood, but not 

 of the bark, which grows inwardly or by a deposition of matter 

 internally, which is the only real endogenous growth. It in- 

 cludes all the trees and shrubs of the temperate zones. 



52. In the embryo state, all plants are composed of cellular 

 tissue. As germination advances, the cells begin to elongate, and 

 form fibers and vessels which penetrate the cellular substance. 

 By multiplying in number, they form a circle of fibro- vascular 

 bundles, about midway between the center and circumference 

 of the young stem. Fig. 23 a represents a magnified section in 



t' * 



a — a section of a young plant greatly magnified ; r r — cellular tissue ; 

 ww— bundles of spiral vessels and woody fiber ; p— the pith. 



b — 7i n— new bnndtefl of spiral vessels and woody iilier, forming a more 

 complete circle. 



which the bundles begin to present a circular outline. As the 

 season advances, the fibro-vascular bundles increase in number 

 (Fig. 23 £>), and the lateral spaces become less and less, and by 

 subdivision more numerous, until they are exceedingly thin and 

 the contained cellular substance is compactly pressed, forming 

 very thin plates called medullary rays. The inner vessels of this 

 zone are true spiral vessels, and they, together with the fibers 

 and cells, form a thin cylinder called the medullary sheath, 

 Fig. 22, m. 



What is the first called ? How characterized ? "What does it include I— 

 52. What takes place as germination advances! How an the medullary 

 n<Ys formed i What constitutes the medullary sheath ? 



