7 8 PLANT LIFE. 



cells, with huge intercellular spaces which are filled with air. 

 The root thus serves to buoy up the parts of the plant to 

 which it is attached. 



90. (d) Tendrils, thorns, etc. — In a very few plants, 

 aerial roots are modified into tendrils , being slender, sensitive 

 to contact, clasping the objects which they touch, if of 

 suitable size, and thus assisting the plant to climb ; in some 

 instances they are altered into thorns, being short, rigid, and 

 sharp-pointed ; in others, being exposed to the light, they 

 develop chloroplasts, which enables them to act as organs for 

 the manufacture of food. 



91. Branching. — Both primary and secondary roots may 

 branch. The mode of branching is of two sorts, either by 

 dichotomy, or by the production of lateral branches. 



92. (a) Dichotomy occurs only in a few fern worts, whose 

 roots possess a single initial cell. In this case, however, 

 the single initial cell (^[ 77) is not divided into two equal 

 parts by a partition-wall, as in true dichotomy (see ^| 103), 

 but the initial of the new branch arises from a very young 

 segment as in Metzgeria (see fig. 61). The result is a fork- 

 ing which cannot be distinguished from a true dichotomy. 



93. (b) Monopodial branching. — In the common mode 

 of branching, the monopodial, the central axis grows most 

 vigorously, and bears lateral branches upon its sides. The 

 normal branches arise from lateral growing points, which 

 originate in regular succession behind the apical growing 

 point. But sometimes branches appear out of this regular 

 order. Such are called adventitious roots. (See ^[ 76.) 



94. Position. — Whether regular or adventitious, the posi- 

 tion of the growing points is determined by the vascular 

 bundles in the stele, since they originate opposite the xylem 

 bundles, or with definite relation to them. (See figs. 92, 

 93.) The number of vertical ranks of branches can, there- 

 fore, be predicted with some certainty from the structure of 



