220 XVII. THE ROOT APEX. 



material. In the growing apex the stratified arrangement of the 

 cell layers can be followed, much as in the stem-apex of Hipjmris. 

 The root-cap of Lemna is not exfoliated, and does not need 

 therefore to be renewed from the interior, a condition which is 

 also shown by numerous other free swimming aquatic plants. In 

 land plants, on the other hand, the root-cap undergoes continuous 

 destruction on its outer side, and must therefore be renewed from 

 the interior. 



Boot-apex of Barley. The structure of the root-apex can be 

 studied with comparative ease in the Grasses (Graminese), and 

 they are fairly typical. They provide us, it is true, with only one 

 of the possible types of root-growth present amongst Phanerogams, 

 but still one widely spread and instructive, and therefore very 

 suited to give us an insight into the processes in question. In 

 order to obtain favourable material, we choose plants removed 

 with care from flower-pots. If we turn the flower-pot upside 

 down so that the whole contents come out bodily, a result often 

 assisted by jerking the flower- pot slightly -down on its rim, the 

 root-apices will be usually found free on the exterior of the ball of 

 earth. For careful study we will select the common Barley, 

 Hordeum vulgar e, and in order to get general information, first 

 prepare a cross-section through an older part of the root. In the 

 middle of the central vascular cylinder or stele, we find a large 

 duct or vessel, and in its periphery about eight xylem rays alter- 

 nating with the same number of strings of bast, As, however, is 

 usual in grasses, the xylem rays extend quite to the endodermis, 

 and therefore interrupt the pericycle. In the endodermis can be 

 seen clearly the cuticularised strips on the radial walls, showing 

 as dark shadings ; outside it comes the thick primary cortex, 



The longitudinal section of the root-apex we prepare between 

 the thumb and forefinger. This can be done well on fresh material 

 only. The section must be sufficiently median ; then the struc- 

 ture is plain enough, even without the use of reagents, though 

 here also Eau de Javelle can be used with advantage (cf. page 210 

 in the previous chapter). Most prominently can be seen that the 

 body of the root is sharply delimited from the root-cap. We can, 

 in fact, follow a line which is prolonged from the outer surface of 

 the epidermis, continuing over the apex, between the body of 

 the root and the root-cap (Fig. 87). Nevertheless the dermatogen, 

 as such, does not pass over the apex, but it can rather be said that 

 the dermatogen (d) and the periblem (b) of the apex come together 



