GROUPS OF PLANTS. 



7i 



leaves, and are usually quite simple in structure. The kaves vary- 

 in form but are usually narrow and somewhat thickened giving 

 them a needle-like appearance. 



In addition sporophylls (spore-bearing leaves) are formed at 

 the ends of the young shoots or in the axils of more mature ones 





s.m 



Pig. 47. Pinus reflexa. Transverse section of a portion from the inner face of the 

 spring wood showing a schizogenous resin duct or passage with the central canal (C) and 

 the thin-walled and resinous epithelium (ep); with parenchyma tracheids (t), the spring 

 wood (Sp. W.) and the summer wood (S. W.). After Penhallow. 



The Coniferae represent the most ancient group in which resin passages or reservoirs 

 are found. While these passages show certain important variations in structure and origin, 

 and while even in certain genera of the group, as in the genus Pinus, they exhibit consider- 

 able variation in detail, yet in this genus they are all of the same structural type as in Pinus 

 reflexa, the white pine of the high mountainous regions of New Mexico and Arizona. The 

 epithelial tissues are thin-walled and readily broken in making sections except in the hard 

 pines as the Loblolly pine (P. tcBda), where the cells often become strongly resinous. (See 

 Penhallow's "Manual of the North American Gymnosperms.") 



(Fig. 51). These are compactly arranged forming cones or 

 strobili which are always of two kinds and borne on different 

 twigs of the same plant or on different plants. The staminate 



