112 



PLANTS, THE FOOD OF ANIMALS 



with roots and rhizoids stretching out in all directions for the 

 absorption of water and salts. The fronds grow up at intervals 

 from the underground stem, a single, vigorous rhizome often 

 bearing several fronds (Fig. 42). 



The Rhizome. The main stem of the underground part may 

 be a simple root-like structure of practically uniform size and 

 several feet in length, or it may be branched, with numerous 

 lateral rhizomes penetrating the earth in different directions. 

 The main rhizome and its branches lie a few inches below the 

 surface and always parallel with that surface, while roots grow 

 out from it into the earth below. In addition to the roots there 



FIG. 43. Branch of a rhizome of Pteris showing the apical bud (a.5.) stumps of 

 numerous leaves (I 1 , I 2 , etc.), and a part of the main rhizome (rh.)\ r, roots. 

 (From Sedgwick and Wilson.) 



are numerous filamentous outgrowths, termed hairs or rhizoids, 

 which differ in finer structure from the roots. One end of the 

 rhizome consists of soft white tissue quite different in appear- 

 ance from the black surface. These are the growing points of 

 the rhizome and branches, all growth of the underground trunk 

 or stem being in a linear direction, and new cells are added by 

 growth of the terminal cell, termed the apical cell (Fig. 43). 



HISTOLOGY 



A cross section of a rhizome (Fig. 44) shows that it is not 

 quite circular in outline, the transverse axis being somewhat 



