148 D. H. SCOTT. 



Other hand, are by no means so far advanced. The absorp- 

 tion of the cross-walls is here only beginning, and some of 

 them are still perfect. The side-walls also are still without 

 perforation. 



In the hypocotyl the hypodermal vessels are just as far 

 advanced as in the root. The cross-walls are absorbed, and 

 a vessel may often be traced a considerable distance without 

 finding any signs of them. The latex-vessels of the fibro- 

 vascular bundles are further advanced than the correspond- 

 ing ones in the root, but not so far as the hypodermal vessels. 

 The cross-walls are perforated, and much larger openings 

 are present in the side-walls. The latter fact, which I have 

 often observed, agrees with Smalhausen's statement, that 

 the absorption of the side-walls precedes that of the cross- 

 walls, though other observations lead me to doubt whether 

 this is an invariable rule. 



In the cotyledons the perforations are smaller than in the 

 hypocotyl. Here all the laticiferous vessels, whether be- 

 longing to the fibro-vascular system or not, show the same 

 stage of development. Their articulations are comparatively 

 short, corresponding to the slow growth of this part. 



The absorption of the cross-walls in the cotyledons con- 

 tinues to make but slow progress. Even in seedlings, with 

 the root about one cm. in length, the process is by no means 

 completed (fig. 5). When two vessels are in contact large 

 perforations already exist in the side-walls. The cross-walls, 

 however, are only partly absorbed, and in some places the 

 openings are still quite small. 



As regards the order in which these vessels are developed 

 in the different parts of the seedling, it appears to me that 

 two rules are followed. In the first place the principle holds 

 good that the latex- vessels reach maturity soonest in the 

 parts where growth first takes place. Accordingly the hy- 

 podermal vessels of the root are completed very early, because 

 the base of the root is the part which has the most energetic 

 growth when germination begins. The corresponding vessels 

 in the hypocotyl are little if at all more backward, for here 

 also elongation begins very early. On the other hand, the 

 cotyledons scarcely grow at all for a long time, and here 

 too the development of the laticiferous vessels goes on very 

 slowly, and is not completed till very late. 



As mentioned above, Schmalhausen called attention to 

 these facts. But, secondly, I have often had occasion to 

 observe that the latex-vessels of the plcrome are very back- 

 ward in their development compared to the hypodermal 

 vessels. This is particularly conspicuous in the root, but 



