LESSON 17.] STEMS OF TREES. 49 



important difference exists between the plants called Endogenous 

 and Exogenous; for in the former, the bundles are dispersed 

 throughout the whole diameter of the plant, without any particular 

 plan, the intervals being filled by cellular tissue. 



290. In the latter they are arranged side by side, so that a hol- 

 low cylinder of wood is formed, which includes within it the pith, 

 whilst itself is enclosed in another (outer) layer the bark. 



291. But there is yet another and lower form of vegetable life 

 Acrogens ; in these the bundles of vessels are simultaneously pro- 

 duced, and the additions to the stem take place at the summit, by 

 the union of the bases of the leaves tree-ferns afford an example. 



292. Thus we have, 



Acrogens (from akros, summit; genncein, to produce), or sum- 

 mit growers. 



Endogens (from endon, within), or inside growers. 



Exogens (exo, outward), or outside growers. 



298. But other and important distinctions still further define 

 these three orders of plants, even in their earliest state. Thus, 

 Acrogens have a cellular embryo, which has no cotyledon (Greek for 

 seed-lobe) ; or, in other words, has no leafy appendages to the young 

 plant ; they are called therefore Acotyledonous (a, without). Endo- 

 gens have an embryo with one cotyledon, and when sprouting send 

 up a single seed-leaf; these therefore are called Monocotyledonous 

 (monos, one). Exogens possess two such seed-lobes, or cotyledons, 

 and are called Dicotyledonous (dis, two). 



294. Consequently, Acrogens are Acotyledonous. 



Endogens are Monocotyledonous. 

 Exogens are Dycotyledonous. 



295. In all parts of the globe, Exogens are by far the most nu- 

 merous of the Vegetable Kingdom ; the forest trees of the World 

 are Exogens, although in warm climates they are found associated 

 alike with Endogens and Acrogens, which, in such climates, are 

 more abundant, and attain greater size, than in more temperate 

 regions. 



296. In its external aspect the Acrogenous stem greatly re- 

 sembles the Endogens / it is unbranched, usually of small, nearly 

 uniform diameter, and produces leaves at its summit (Fig. 85). 



297. The Tree Ferns, which furnish the best example of this 

 kind of stem, are met with only in hot climates. 



In India, they present a stem from six inches to eight inches in 

 diameter, and attain a height of from fifty to sixty feet. 

 4 



