SM MESSES. CTEIL WEST AlsB II. TAKEDA 0^' 



to another. There is very little evidence in favour of the view tbat the parenchyma 

 of the secondary cortex takes over the function of phloem. Although it is usual for 

 the xylem of aquatic plants to undergo reduction to a greater or lesser degree, we know 

 of no other genus, either amongst Vascular Cryptogams or amongst Phanerogams, in 

 which the phloem of the stem is reduced to the point of extinction. On the other hand, 

 many aquatics possess a well-developed phloem *. 



As a matter of fact, the tissue cut off internally by the cambium is largely composed 

 of sieve-tubes f which are very similar in form to the parenchymatous cells, from which, 

 however, they can easily be distinguished by the presence of definite sieve-areas and 

 frequent deposits of callus and by the absence of starch-grains. 



The sieve-areas, which are of the typical cryptogamic type, occur on the radial, 

 tangential, and horizontal walls of the sieve-tubes (PI. 34. fig. 13 ; PI. 35. figs. 18, 19 ; 

 PI. 38. fig. Q4i ; PI. 40. figs. 74, 76) ; hence it is strange that their presence should have 



41 



been overlooked or that they should have been mistaken for secondary xylem elements 

 in process of lignification. Many of these sieve-tubes have comparatively thick cell- 

 walls divided by irregular reticulate thickenings into numerous sieve-areas ; it is 

 these cells with irregularly thickened walls which appear to have been mistaken for 

 secondary xylem elements by Wilson. Smith and by Miss Stokey. 



As mentioned above, deposits of callus giving positive reactions to Corallin-soda 

 and other recognized ' callus ' stains are very frequently met with in these cells (PL 35. 

 figs. 18, 19). Occasionally this substance is deposited in such quantity as to 

 completely fill the lumen of the cell (PI. 40. figs. 74, 76), especially m the sieve-tubes 

 first cut off by the cambium. A small nucleus is sometimes found in these elements, 

 but, when present, the nucleus is always much smaller than the nuclei of neighbouring 

 parenchymatous cells and is probably undergoing degeneration. Thus, it is evident that 

 the sieve- tubes of the secondary phloem of Isoetes japonica are essentially similar in struc- 

 ture to those found in other Vascular Cryptogams, differing only in shape from the usual 

 type. It has already been shown by Seward and Pord (29) that the so-called *querge- 

 streckte Zellen' of Zenetti (37), which occur in the stem of Osmtm da and Todea, are 

 actually sieve-tubes, which are considerably elongated in the horizontal {i. e. tangential) 

 direction. The description of these peculiar cells given by Seward and Ford (l. c. p. 245 

 and PI. 29. fig. 40) would serve equally well for the sieve-tubes of Isoetes. Moreover, 

 these authors [l. c. p. 242) remark that in the apical region of the stem of Todea these 

 cells contain large nuclei. 



We have invariably found that on the phloem {L e. lower or abaxial) side of the 

 leaf-trace the sieve-tubes of the secondary tissue are in direct organic connection with 

 those of the leaf -trace, as is clearly shown in PI. 35. fig. 21. Moreover, a gradual 

 transition from the extremely short sieve-tubes of the secondary phloem to the 



* E. g., Potamogeton, CeratojiJiyJhirn, Eloclea^ etc. 



t Strictly speaking, these elements are sieve-c^7?s and differ from the sieve-tubes of the leaf- and root-traces, 

 which consist of rows of cells (= « members ' of De Bary, 12, p. 172). 



