ISOETES JAPONIC A, A. BE. 



339 



however, it bears only a very superficial resemblance. Whereas the leaf -rudiments 

 possess a thin layer of cuticle, the conical protuberance which constitutes the apex of 



the stem has no cuticle. 



It is impossible to distinguish any definite apical cell amongst the superficial cells of 

 this protuberance, all of which, as Bruchmann (7, p. 568) correctly observed, are of 

 similar shape and size. These cells usually divide by anticlinal walls, although occasional 



periclinal divisions are found. 



In the underlying tissue ('Urmeristem' of Bruchmann), the cells of which are 

 irregularly arranged, anticlinal and periclinal divisions occur in equal numbers. The 

 relatively large parenchymatous cells lying immediately below the protuberance arc 

 arranged in more or less regular series (PL 39. figs. 67, 60, 70) and constitute a poorly 

 defined plerome, the cells of which, probably owing to the peculiar habit of the plant, 



are not vertically elongated. 



Traced still further downwards, a certain number of these large cells are found in all 

 stages of lignification (PL 34. figs. 10, 12). These lignified elements form the trachcids 

 of the stem-stele. Scott and Hill (28, p. 420, PL 23. fig. 6) follow Van Tieghem in 

 ascribing a centripetal development to these lignified elements, but in every one of our 

 preparations of I. japonica the development of these tracheids is obviously centrifugal 

 (PL 34. fig. 12). It is, perhaps, inadvisable to lay great stress on this point, since a 

 certain amount of irregularity inay be expected in such a slow-growing organ. 



The remaining cells retain their meristematic condition for a considerable period and 

 ultimately give rise to (1) the numerous small parenchymatous cells (xylem parenchyma), 

 which with the tracheids constitute the xylem of the stem-stele, and (2) a zone of 

 parenchyma which completely surrounds the xylem except where interrupted by the 

 outgoing leaf-traces. This peripheral zone of parenchyma occurs as a definite structure 

 in every species examined ; we therefore propose to call it the ' parenchymatous mantle * 

 (text-figs. 1, 2 ; PL 34. figs. 10, 13 ; PL 38. fig. 62). 



The sieve-tubes of the primary phloem are usually dilferentiated from the central 

 cells of this zone of parenchyma, in w^hich case they are surrounded both internally 

 and externally by one or more layers of parenchyma (PL 34. figs. 10, lo). But 

 occasionally the more peripheral cells of the parenchymatous mantle become difler- 

 entiated as sieve-tubes, and since the cambium arises from the outermost layer of the 

 plerome (i. e. outermost layer of the * parenchymatous mantle '), it follows that the 

 sieve-tubes of the primary phloem will then abut directly on the tissues which result 

 from the activity of this cambium (PL 35. fig. 21 ; PL 38. fig. 64). 



We therefore agree with Scott and Hill (28, p. 423) and Stokey (32, p. 320} that 

 the tissue in immediate contact with the primary tracheids is always parenchymatous . 

 as to whether this parenchyma belongs to the xylem rather than to the phloem we 

 prefer to leave an open question, but we usually find that near the apex of the 

 stem, the xylem tracheids are difl'erentiated before the sieve-tubes of the primary 

 phloem (text-fig. 1; PL 34. figs. 10, 12). 



