150 



ANATOMY OP ENDOGENOUS 



[LESSON 25. 



almost entirely of soft and tender cellular tissue. But as they grow, 

 wood begins at once to be formed in them. 



422. This woody material is arranged in the stem in two very 

 different ways in different plants, making two sorts of wood. One 

 sort we see in a Palm-stem, a rattan, and a Corn-stalk (Fig. 351) ; 

 the other we are familiar with in Oak, Maple, and all our common 

 kinds of wood. In the first, the wood is made up of separate threads, 

 scattered here and there throughout the whole diameter of the stem. 

 In the second the wood is all collected to form a layer (in a slice 

 across appearing as a ring) of wood, between a central cellular part 

 which has none in it, the Pith, and an outer cellular part, the Bark. 

 This last is the plan of all our Northern trees and shrubs, and of the 

 greater part of our herbs. The first kind is 



423. The Endogenous Stem ; so named from two Greek words mean- 

 ing " inside-growing," because, when it lasts from year to year, the 



new wood which is added is interspersed among 

 the older threads of wood, and in old stems the 

 hardest and oldest wood is near the surface, and 

 the youngest and softest towards the centre. All 

 the plants represented in Fig. 47, on p. 19, (ex- 

 cept the anomalous Cycas,) are examples of En- 

 dogenous stems. And all such belong to plants 

 with only one cotyledon or seed-leaf to the em- 

 bryo (32). Botanists therefore call them Endoge- 

 nous or Monocotyledonous Plants, using sometimes 

 one name, and sometimes the other. Endogenous 

 stems have no separate pith in the centre, no distinct bark, and no 

 layer or ring of wood between these two ; but the threads of wood 

 are scattered throughout the whole, without any particular order. 

 This is very different from 



424. The Exogenous Stem, the one we have most to do with, since 

 all our Northern trees and shrubs are constructed on this plan. It 

 belongs to all plants which have two cotyledons to the embryo (or 

 more than two, such as Pines, 33) ; so that we call these either 

 Exogenous or Dicotyledonous Plants (16), accordingly as we take 

 the name from the stem or from the embryo. 



425. In the Exogenous stem, as already stated, the wood is all 

 collected into one zone, surrounding a pith of pure cellular tissue in 

 the centre, and surrounded by a distinct and separable bark, the 



FIG. 351. Section of a Corn-stalk (an endogenous stem), both crosswise and lengthwise. 



