72 STRUCTURAL BOTANY 



The pore opens when the guard-cells become more 

 curved, so that the space between them is widened. 

 It closes when they straighten, so that their sides are 

 brought into contact. We shall see later on how this 

 opening and closing of the stomata works in with the 

 mode of action of the leaf as a whole (see p. 221). 



c. Structure of the Root 



We will now examine the structure of the main 

 root or tap-root, which forms the direct downward 

 continuation of the stem. It does not differ essentially 

 from its branches, but has the advantage of being 

 rather larger, and more convenient for investigation. 



In order to find the primary structure of the root 

 unaltered, we must take a very young one ; for in the 

 Wallflower, and most members of its Class, the cambium 

 begins its activity early, and soon brings about a com- 

 plete change in the distribution of the tissues. 



We will start as before with the vascular system, 

 which at first sight appears very different from that of 

 the stem. The middle of the root is traversed by a 

 slender cylindrical strand of vascular tissue, which 

 runs in a straight course from base to apex, and is 

 directly continuous with the bundle-system of the 

 stem. In a root which has reached the stage shown 

 in transverse section in Fig. 28, the xylein forms 

 a plate of tissue passing through the centre of the 

 vascular cylinder; the phloem is grouped in two 

 distinct bundles, one on each side of the xylem-plate. 

 If we had examined a still younger root, we should 

 have found that only the cells at the two ends of the 



