346 MESSES 



ON 



tissues of the stem-stele. These secondary tracheids are very similar to those of the 

 primary xylem and usually occur in groups (PL 38. fig. 62), although single elements 

 are occasionally found. The reticulate thickenings of the walls of these elements are 

 lignified ; hence, in sections stained with, the Safranin-Hsematoxylin combination, they 

 stand out sharply from the surrounding secondary tissues, which consist of sieve-tubes 

 and parenchyma. Moreover, these secondary xylem elements have no contents. 



4. Secondary Cortex. 



It is generally agreed that the tissue cut off externally by the cambium consists, 

 in all the species of Isoetes hitherto examined, of typical parenchymatous cells, which 

 form the relatively bulky secondary cortex. These cells, whicb are produced very 

 rapidly, are at first arranged in radial rows and form a compact homogeneous tissue. 

 But, sooner or later, as they move farther and farther away from the centre of the 

 stem, their arrangement becomes correspondingly less and less regular, whilst large 

 intercellular spaces appear between them. These cells contain numerous starch-grains 

 and proteid granules, but oil-drops were never observed. 



The walls of the peripheral cells of the secondary cortex become suberized and form 

 a feebly developed periderm. 



It is somewhat difficult, even in young specimens, to distinguish the cells of the 

 primary cortex from the parenchymatous cells formed by cambial activity, since they 

 sbow a general agreement in size, shape, and contents. 



Apical Meristem and Development oe the Primary Tissues or the Ehizophore. 



The results obtained by ns from a careful study of the rhizophore region of the 

 caudex of I. japonica and of L lacustris, cannot be reconciled with the conclusion 

 arrived at by Scott and Hill with reference to the development of the same region 

 of J. Hystrix. These botanists (28, p. 431) give the following explanation of the 

 mode of growth of tbe rhizophore : " the downward growth of the base of the stem 

 is entirely due to the activity of the cambium, and to the addition of new root-bases." 

 And {I. c. p. 441), " The wood, which is added at the bottom of the stele, and which 

 remains functional when the primary wood higher up is disorganised, is entirely built 

 up by the sympodium of root-bases." 



It has already been shown that growth of the stem-stele of /. japonica is acropetal, 

 fresh xylem and phloem-tissue being added continuously by tbe slow and gradual 

 difi'erentiation of large plerome cells derived from the apical meristem. The growth 

 of the rhizophore-stele takes place in much the same way ; that is to say, the growth 

 of the rhizophore-stele is also acropetal, fresh xylem and phloem-tissue being gradually 

 differentiated from the primary meristem ♦, which, however, differs from that of the 



• This 



(c/. Hofmeister, 16, Taf. 48. fig. 3), persists throuffhout the life 



origiu and in appearance from the true cambium. 



distinguish 



with 



