186 



SYSTEMATIC BOTANY. 



be split clown one side and spread out into a plane, an 

 appearance approximating more or less that in the accom- 

 panying diagram (Fig. 298) will be presented. The 

 distribution of the bundles or midrib and veins in the 

 leaves is such as to form a distinct net-work ; and the 

 leaves of the Dicotyledones, or Dicotyls, are, therefore, said 

 to be reticulate, or netted-veined. 



igi. The epidermis of young 

 shrubby or arboreous plants is 

 later replaced by cork. This may 

 develop in a continuous layer, so 

 as to form a periderm ; or corky 

 plates or layers may arise, sit- 

 uated, more or less deeply, in the 

 I I cortical tissue. That external to 



I these corky layers looses its vital- 



ity, and a thick bark may appear, 

 consisting of this firm, dry tissue, 

 and the phloem, or bast, which lies 

 immediately below. This, in the 

 outer exposed part, is constantly 

 peeling or falling off, or being worn 

 off by weathering ; but remains a 

 constant thickness, due to the fact that it is ever replen- 

 ished within by the cambium. The flowers are generally 

 four or five-parted ; that is, each floral whorl contains four 

 or five members. The embryo and germination of a 

 Dicotyl is shown in Fig. 277. The sub-class is very large, 

 embracing all of our forest trees except the ConifercB 

 (Pines, Firs, etc.), and divisible into three groups, namely, 

 Apetalcd, GamopetaloB, and Pohjpctalac or Ghoripdalm. 



Fig. 208. Diagram, showing llic course of the fibro-vascular bundles in an exoge- 

 nous slcm (of Slachys) ; the figures indicalc the position of the leaves. 



