BoT— Vol. I.] CAMPBELL— NATAS AND ZAWVICILELL/A . 31 



IX. The Stem. 



The origin of the stem is, as we have seen, lateral, and it 

 can be readily traced to the upper half of the middle seg- 

 ment of the young embryo. The projection of the apex is 

 brought about by an acceleration of the growth in the der- 

 matogen and plerome at one point of the segment, which 

 soon results in the development of an evident bluntly conical 

 protuberance, which grows rapidly and assumes a more 

 pointed form. A longitudinal section of the embryo at this 

 time (fig. 70, a) shows that the plerome cylinders of the 

 stem, leaf, and root are continuous, forming a single axial 

 strand ; but at the point where the stem-apex is forming there 

 is an enlargement of the plerome, which is beginning to form 

 a branch going into the stem. Very soon after this, the 

 stem-apex shows the same arrangement of tissues as is found 

 in the fully developed embryo. 



X. The Root. 



The primary root of the Naias differs in several import- 

 ant points from that of the other Monocotyledons that have 

 been described. In the first place, the bulk of the root, at 

 least the whole of the plerome and- part of the periblem and 

 epidermis, are parts of the same segment of the embryo as 

 the stem, while the plerome initials and terminal epidermis 

 alone are formed from the lowest of the primary embryonic 

 segments. Another very important difference is the com- 

 plete absence of a root-cap. In none of the specimens ex- 

 amined was there the slightest trace of this structure, unless 

 we can regard the cells at the apex of the root, which are 

 continuous with the epidermis of the rest of the embryo, as 

 such. Unfortunately Hofmeister's paper, where the embry- 

 ogeny of Naias majo?' is described, was not accessible, and 

 so we cannot now make a comparison with that species; but 

 no mention of the absence of a root-cap in JVaias major is 

 made in any of the references to Hofmeister's paper exam- 

 ined by me. The separation of the root from the stem does 

 not occur until after the differentiation of the tissue-systems 



