STEMS OF EXOGENS. 23 



29. The CLASS OF EX'OGENS comprises all the trees and 

 shrubs of our forests, and is composed of vascular plants, the 

 stem of which has a medullary canal in the centre, and grows by 

 super-posed layers (Jig- 13). 



30. The CLASS OF EN'DOGENS comprises those plants in which 

 the stern has neither a central canal nor concentric layers (fig* 

 14). The palms belong to this division. 



Structure of the Stems of Exogenous Plants. 



31. In the stems of these plants we distinguish two principal 

 parts : the bark, and the central, or ligneous part, which might 

 be called the body of the stem. Each one of these portions is in 

 turn composed of several different parts ; the central portion of 

 the stem is formed by a central pith, by ligneous layers, and by 

 medullary rays ; the bark, or cortical portion, is composed of the 

 epidermis of a cellular envelope, and of a fibrous part named 

 liber, or cortical layers. (Liber, Latin, bark, is the interior 

 lining of the bark of ex'ogenous plants.) 



32. If we cut through an elder, or any other ex'ogenous tree, 

 transversely, we observe in the centre a canal, which is ordi- 

 narily angular, or very nearly cylindrical, and which, in the 

 young branches, if not in the whole plant, is filled with a round 

 cellular tissue (Jig- 13, a); this cavity is called the medullary 

 canal, and the cellular tissue found in it is named the pith of the 

 plant. 



33. This central pith is of a soft consistence, and of a very 

 homoge'neous* structure; while young it is always humid, and 

 of a light greenish tint ; but with the progress of age, the cells 

 of which it is composed become empty, dry, and assume a re- 

 markable whiteness ; sometimes it is torn by the effect of the 

 elongation of the stem, and separates in laminae or bundles, as 

 may be easily seen in branches of jasmine that have attained one 

 year old. 



known by the veins of their leaves running parallel with each other, with- 

 out branching or dividing. Grasses, lilies, the asparagus, and similar 

 plants belong to this class, which in warm countries contains trees of large 

 size, such as palms and screw pines. 



* Homoge'neous. From the Greek, omou, together, and genas, kind. 

 Of the same kind. Bodies whose constituent elements are of one and the 

 same kind, are said to be homoge'neous. 



29. What is the general character of those plants which constitute the 

 class of Ex'ogens ? 



30. What kind of plants does the class of En'dogens comprise ? 



31. How is the stem of ex'ogenous plants divided ? What is the central 

 portion ? What is bark ? 



32. What is the medullary canal of plants ? What is meant by pith ? 



33. What is the character of pith ? 



