234 HOW CROPS GROW. 



of plants, interpenetrating the cellular tissue. There are 

 several varieties of ducts, viz., dotted ducts, ringed or an- 

 nular ducts, and spiral ducts, of which illustrations will 

 be given when the minute structure of the stem comes 

 under notice, fig. 49, p. 271. 



The formation of vascular tissue takes place by a simple 

 alteration in cellular tissue. A longitudinal series of ad- 

 hering cells represents a tube, save that the bore is ob- 

 structed with numerous transverse partitions. By the 

 removal or perforation of these partitions a tube is devel- 

 oped. This removal or perforation actually takes place 

 in the living plant by a process of absorption. 



CHAPTER IIL 



THE VEGETATIVE ORGANS OF PLANTS. 



1- 



THE ROOT. 



The ROOTS of plants, with few exceptions, from the first 

 moment of their development grow downward, in obe- 

 dience to the force of gravitation. In general, they require 

 a moist medium. They will form in water or in moist cot- 

 ton, and in many cases originate from branches, or even 

 leaves, when these parts of the plant are buried in the 

 earth or immersed in water. It cannot be assumed that 

 they seek to avoid the light, because they may attain a 

 full development without being kept in darkness. The 



