512 RET. GEORGE HENSLOW ON A THEORETICAL 



We thus ?ee that the form of vessel described above is of a 

 degraded or arrested type, and that on the one hand it is the 

 result of water, or parasitism *, or saprophytism, and on the other 

 it is characteristic of endogens generally, in which it has become 

 an hereditary feature handed down from the time when the 

 ancestors of existing endogens were aquatic t. 



Origin of the Endogenous arrangement of Cords. — I would here 

 venture to suggest a possible origin of the normally scattered 

 arrangement of the fibro-vascular cords of endogenous stems. It 

 will be seen from the figure given by De Bary of Nelumbium £, 

 that the cords in a long internode are arranged in concentric 

 circles. The five outer zones have been traced into leaves : some 

 obscurity remains with regard to the inner circles. Now, when 

 we remember that such simple herbaceous flowering-stems a3 those 

 of Anemone, a Daffodil, and Alocasia odora § have their cords 

 arranged in more than one zone, we begin to suspect that the 

 zones of Nelumbium represent a first stage of disintegration of 

 the compact zone or xylem-cylinder characteristic of exogens. 

 This last is of course correlated with the requirements of 

 strength; but this is not necessary in submerged or subterra- 

 nean rhizomes. "With these are correlated degradations (such as 

 M. Costantin brought about by experiments). The order of 

 retrogression, therefore, may be speculatively suggested to be 

 as follows: — First, the cords of the xylem-cylinder remain 

 isolated, no interfascicular cambium completing the zone ; 

 secondly, a second series of cords forming another zone belong- 

 ing to the next sheathiug-leaf, instead of uniting by intercala- 

 tion with the first, forms a second circle, then a third would 

 follow; and a fourth, and so on. The result would be a series 

 of concentric circles, as seen in Nelumbium. Moreover, the leaf- 

 traces consist of groups of five each (in Nymphcea), three to 

 four groups being seen in a transverse section, the lateral cords 

 reaching diametrically opposite points ||. The last and easy 



ssels as occurring in parasites; 



Chatin figures many cases of imperfect 

 only the individual cells are usually "barred" or subscalariform instead of 

 annular. See, e g-, those of Hydnora, pi. xcii. (bis), vol. ii. of his Anat. Com p. 

 des Veg. 



t De Bary s Com p. Anat. p. 369 ; cf. pp. 277 and 340. 

 X Comp. Anat.&c. fig. 112, p. 255. 



§ " Eecherches sur la Structure des Aroidees," V. Tieghem, Ann. des Sci. Nat 

 5 Be>. torn. vi. p. 05, 186G, pi. i. fig. 5. 



| De Bary, Comp. Anat. &c. p. 252. , 



