There is no group of plants among pteridophytes whose anatomy has occasioned so much discussion as the genus Isoetes. The most recent writers, FARMER on I. iacustris, R. WILSON SMITH On I. echiinospora, and SCOTT and HILL on I. Hystrix, disagree in many points. In view of the lack of harmony, both of observation and interpretation, it has seemed advisable to make a comparative investigation of the anatomy of several American species.
HistoricalThe literature on the anatomy of Isoetes began in i840 with a paper by VON MOHL (24).Although he recognized the lycopod affinities of Isoetes, he noted several important points of difference, in the arrangement and structure of the roots, and in the nature of the cambium products. Ever since that day the question of the nature of the cambium activity has afforded a fertile field for discussion, and it is with that subject that this pa-per is chiefly concerned. VON MOHL regarded the whole of the secondary growth as parenchymatous, and states that, as in other vascular cryptogams, there is no increase in thickness of the wood. The next significant work was that of HOFMEISTER (I6) in i857. In discussing the cambium products he says (p. 36i): "The effect of the yearly renovation of the cambial layer is not only to increase and renew the cortical tissue, but new spiral cells also become added, although only sparingly, to the wood of old vigorous plants. Individual cells of the cambium, separated by two or three cambium cells from the older principal mass of the wood, often exhibit thickenings of the walls, which by their delicacy and want of color betray their undoubted recent origin." In i873 appeared Russow's Vergleichende Untersuchungen der Leitbiindel-Kryptogamen (i8), with a discussion of the anatomy of I. lacustris, I. echinospora, and I. Hystrix. This paper has had great influence on subsequent work, and to Russow's work may be attributed the cur-II] [Botanical Gazette, vol. 47 This content downloaded from 044.224.250.rent conceptions of the nature of the cambium products, i. e., that the cambium produces cortex externally to itself, but internally it produces a tissue which he calls the "prismatic layer," and this he regards as being made up of phloem cells, tracheids, and parenchyma. HEGELMAIER (12), writing in i874, is inclined to question Russow's interpretation, but adds that no decisive grounds can be given against the interpretation of the Dauerzellen as soft bast. FARMER (9) published in i899 a very full discussion of I. lacustris. He seems reluctant to accept Russow's theory of the nature of the cambium products, but does not offer another. After quoting Russow he adds: "Quite apart from the fact that it is produced internally to the cambium and would, from the point of view of its position, be anomalous, its structure is remarkably complex and heterogeneous." In discussing the structure of the "prismatic layer" he says:The zone-like arrangement consists in alternations of tubular thin-walled cell-rows of varying thickness, whose cell contents are clear and water...
