MATERIAL FOR STUDY OF THE ROOT 135 



a great deal of attention because of the apparent physiological 

 importance of the endodermis as a cellular layer which regulates 

 the entry of water and mineral salts into the vascular system of 

 the stele. The assumption is made that the suberized nature of 

 the Casparian strip renders it impermeable to water so that 

 diffusion must take place through the tangential walls and the 

 protoplasts of the endodermal cells, i.e., through a semi-perme- 

 able membrane. However, the exact function and significance 

 of the endodermis is in need of much further investigation. 



In the endodermis of Ranunculus, the presence of the Cas- 

 parian strip is indicated by the red color of the short radial walls. 

 Individual cells of the endodermis may have uniformly and 

 heavily thickened walls, a phenomenon previously recorded by 

 Caspary in the case of Ranunculus Ficaria (ef. De Bary, p. 123). 

 In the roots of certain plants, all of the cells of the endodermis 

 may be thick- walled in character (cf. Eames and MacDaniels, 

 p. 102, Fig. 51). Within the endodermis occurs a single layer 

 of living cells which is known as the pericycle. The most con- 

 spicuous portion of the stele is represented by the primary vascu- 

 lar tissue which shows the characteristic radial arra7igement of 

 the xylem and phloem goups. It will be seen that four radial 

 plates of xylem (which join in the center of the root, thus form- 

 ing a protostele) are present; the stele is therefore designated 

 as tetrarch. The protoxylem (which presumably consists of 

 annular and spiral elements) is represented by several very small 

 "polygonal," thick-walled cells found at the outer edge of each 

 xylem plate. The metaxylem (which probably is composed of 

 pitted elements) consists of much larger cells and as stated previ- 

 ously forms a homogeneous tissue in the center of the root ; a pith 

 is thus absent. Laterally adjacent to each xylem plate near its 

 outer edge, you will find one or two rather thick- walled cells, 

 which are approximately hexagonal as seen in transverse section 

 and which appear distinct from the xylem because of their 

 lighter-staining walls and the possession of a more or less disin- 

 tegrated protoplast. The shape and structure of these cells sug- 

 gest that they are sieve-tubes ; their exact morphology is difficult 

 to interpret but they seem to belong to the phloem. Each of the 

 four phloem groups is sepai-ated laterally and internally from 



