6 Weiss, Root-apex and Young Root of Lyginodcndron, 



hedral apical cell. In older parts of the root, of course, 

 the number of endodermal cells becomes materially 

 increased, but in young roots, where the number six is 

 exceeded, nine and fifteen are not uncommon though they 

 do not always run in multiples of three. But this number 

 six, a little way behind the apex, is what one would expect 

 in a root with a three-sided apical cell, though it might 

 also occur in a root with a group of three cells, which is, of 

 course, also a possibility. The cells of the central cylinder 

 are very small compared with those of the outer layer. 

 In the transverse section under consideration they are 

 somewhat defective on the upper side of the section, where 

 only the most centrally lying cell is clearly visible, but on 

 turning the slide over the other walls can be more or less 

 clearly seen. It will be seen from the drawing of them 

 made with the camera, that they are somewhat irregularly 

 arranged. They are only small in number, and their general 

 arrangement is more reminiscent in this early stage of 

 such a fern root as Azolla rather than of Aspidium. 



In the young stage represented in Fig. 3 no vascular 

 tissue is formed, but it becomes differentiated very 

 early, one protoxylem group making its appearance 

 first, to be followed shortly by a second one {Fig. 4), the 

 two groups becoming subsequently connected by meta- 

 xylem elements. Both these stages are frequently met 

 with among the sections of delicate rootlets. Triarch 

 arrangement is only fijund in rootlets of greater diameter. 

 Whether triarch roots arise from the delicate diarch ones 

 it would be difficult to say ; but in one of the smaller 

 triarch roots there were indications of one of the proto- 

 xylem groups having arisen later than the two others. In 

 very young roots the tissues of the central cylinder are 

 often defective, and frequently only a single or a couple of 

 tracheids are found adhering to the inner wall of the 

 endodcrmis, the other cells having perished {Fig. 4). 



