TSOETES JAPO^'ICA, A. BR. 



more elongated sieve-tubes of the leaf-traces was observed. The sieve-tubes of the 

 basal region of the leaf-traces are remarkably short considering the length of the 

 leaves themselves. 



A possible explanation of the peculiar form of the sieve-tubes of the secondary phloem 

 of Isoetes may be deduced from the very slow stunted growth of this plant. It is 

 generally agreed that some sort of correlation exists between the actual rate of growth 



of a plant-structure and the length of its individual conducting elements. Numerous 

 examples of this correlation might be given ; for instance, the segments or members 

 of the sieve-tubes of certain tropical lianes a.ttain a length of more than 2 mm. (e. g. 

 Calamus Rotang ; JBignonia spp., etc.). 



Isoetes lacustris was examined for comparison with /. japonica. Short, box-like 

 sieve-tubes, similar in every respect to those described above, were found in the internal 

 secondary tissue (PI. 35. fig. 20) associated with starch-containing parenchymatous cells 

 and with secondary xylem elements. In this species also, the sieve-tubes of tlie 

 secondary tissue are in organic connection with those of the leaf -traces (PI. 35. fig. IG). 



In I. japonica the parenchymatous cells {i. e. phloem- parenchyma) cut off internally 

 by the cambium can easily be recognized by the presence of a relatively large 

 nucleus and numerous starch-grains. The cells labelled ph.p. in PL 38. fig. 64 are 

 of this nature, although at first sight they might easily be mistaken for the actual 

 cambial cells; the latter, however, as previously pointed out by Scott and Hill (28, 

 p. 424), do not contain starch-grains. 



b 



These parenchymatous cells are slightly less numerous than the sieve-tubes ; they are 

 always irregularly distributed through the secondary tissue, and, even in very old 

 specimens, are never arranged in definite zones, as described for several other species 

 of Isoetes by Hegelmaier (15, p. 501), Farmer (14, p. 42), and Stokey (32, p. B23). 

 Since the secondary tissue completely envelopes the primary stele, it follows that the 

 cells of the secondary tissue come into contact with the tissues of the leaf-trace as 

 it passes out from the central mass of primary tissue. On the xylem {i. e. upper or 

 adaxial) side of the trace a number of parenchymatous cells occur, and these, as already 

 . described, connect up directly with those of the ' parenchymatous mantle.* 



3. Secondary Xi/lcm. 



No secondary xylem was found in J. japonica ; apparently it does not occur in this 

 species, since plants of all ages were examined. Secondary xylem has been described 



r 



in several other species of Isoetes, namely : — 



I. lacustris (Hofmeister, 16, p. 361; Russow, 26, p. 139); 



L Durieui (Hegelmaier, 15, p. 504) ; 



I. Hfjsti^ix (Scott and Hill, 28, p. 421) ; 



I. echinospora and I. Engelmanni (Wilson Smith, 30, p. 227) ; 



/. Nuttallii and I. mela/nop od a (Stokey, 32, p. 326). 



In Isoetes lacusti^is xylem elements occur spasmodically throughout the secondary 



3d2 



^^ 



