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. 
81. 
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 
