LEAVES AND THE RHIZOME. 



71 



axis or of the branches. Behind each mature leaf remnants of 

 the leaves of preceding years are often to be found, alternating on 

 the sides of the rhizome in regular succession, and showing various 

 stages of decay. The first of these (which is on the opposite side 

 of the rhizome from the living leaf) was alive the previous year ; 

 the next (on the same side with the living leaf) is the leaf of the 

 year before that ; and so on. Fig. 34 shows an example of this 

 sort. The leaf of the present year, l\ is fully developed ; and the 





FIG. 34. (After Sachs.) Branch of a rhizome of Pteris, showing the apical bud (a. 6), the 

 stumps of a number of successive leaves (P, Z 2 , I 3 , etc.), and a part of the main rhizome 

 (rli) ; r, root. 



relics of the leaves of the preceding years are indicated at F, l\ 

 etc. ; I 1 is the rudiment of next year's leaf. 



Internal Structure of the Rhizome. The rhizome is a nearly 

 solid mass, consisting of many different kinds of cells, united into 

 different tissues, and having a very complicated arrangement. Its 

 study is somewhat difficult. Nevertheless the arrangement of the 

 cells is definite and constant, and merits careful attention, since 

 it has many features which are characteristic of the cellular struc- 

 ture of the stems of higher plants. We shall first examine its 

 more obvious composition as displayed in transverse and longi- 

 tudinal sections, afterwards making a careful microscopical study 

 of the cells and tissues. 



Seen with a hand-lens or the naked eye, a transverse section of 

 the rhizome (Fig. 35) presents a white or yellowish background 

 bounded by a black margin (the epidermis) and marked by various 



