PRINCIPAL GROUPS OF PLANTS. 



107 



ferns where the sori are borne on the under surface of the leaves. 

 The microsporangia vary in number from 2 to 15, and are pro- 

 tected in various ways, either being sunk in the tissues of the sporo- 

 phyll, as in Pinus and Abies, or they are, as in Juniperus and 

 Thuja, provided with a covering resembling the indusium of the 

 sori of the ferns. The walls are variously thickened and on drying, 





s.w 



FIG. 68. Pinus rejtexa. 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 Conifera? represent the most ancient group in which resin passages or reservoirs 

 are found. While \hese 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. Tceda), where the cells often become strongly resinous. (See 

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



owing to unequal tension, the sacs are ruptured longitudinally 

 and the spores scattered. The microspores are very numerous, 

 sometimes forming powdery deposits. They are either i -celled 

 or 3-celled. In the latter case two lateral cells act as wings for 

 the dispersal of the spores by the wind (Fig. 69, D). 



