490 REV. GEORGE HENSLOW ON A THEORETICAL 



aquatic submerged stem the cords of the xylem remaiu sepa- 

 rated as isolated masses, while in the terrestrial plant they are 

 united into a compact and continuous zone of wood, though the 

 approximation to an endogen does not proceed further in this 

 c.ise, except in a general degree of degradation. 



Again, though ordinary endogenous stems are easily dis- 

 tinguishable from exogenous, in some cases there still remain 

 somewhat pronounced resemblances. Thus in Taccacece, Dios- 

 corece, and Smilacete, it has been observed that the fibro-vascular 

 bundles are arranged with a tolerable degree of regularity around 

 a central pith. Again, Mobins notes " that both the structure 

 and arrangement of the fib ro- vascular bundles of German 

 orchids show a singular approach to exogens. The bundles are 

 in a single ring in the uppermost portion of the stem, and show 

 at least traces of an active cambium." * 



Histological Elements. — Besides these morphological facts, 

 histology points to a common descent, in that all the details of 

 the tissues are of the same kind, respectively — parenchyma, pros- 

 enthyma, spiral and annular vessels, &c. Each and all can be 

 paralleled in the two classes. 



One marked difference lies in the relative degrees of activity 

 of the peri cycle, which adds to the increase of the diameter of the 

 stem of Draccena t, &C, in an analogous manner to that of the 

 cambium ring in exogens ; but it is now recognized to be much 

 more active in the stems of exogens than formerly, inasmuch 

 as it is the source of the bast or true liber, and many other 

 forms of tissue within the cortex, outside the position of the 

 phloem i. 



Lastly, the anatomical details of leaves taken generally are 

 the same in both classes, though the distribution of the ribs and 

 veins may be different. Even with these, resemblances are not 

 wanting, as in the group Lindley called Dictyogens among 

 endogens. 



are 



Quoted from Journ. Roy. Micr. Soc. 1887, p. 428. I have not seen the 

 original paper. 



t Sachs regards this as an active zone of fundamental tissue, but it is on the 

 " site " of the pericycle; and that the growth is due to this layer is supported 

 by analogy, as in radishes, beetroot, &c. M. Ph. van Tieghem attributes it to 

 the pericycle. 



J See M. Morot's paper, Ann. des Sci. Nat., But. 7 ser. xx. 1885, p. 234. 



