On Secondary Thickening in Pteridophyta. 213 
ISOETACEAE. 
Literature. 
Von Mohl.—Uebcr den Bau des Stammes von Isoetes lacustris. 
Vermischte Schriften. Linnaea, 1840. 
Hofmeister.—Bcitriige zur Kenntniss der Gefiiss-Kryptogamen. 
I. Die Entwickclungsgcschichte der Isoetes lacustris. Abhand. 
der K. Sachs, Ges. d. Wissensch., IV., 1852. 
Russow.—Vergleichende Untersuchungen, St. Petersbourg, 1872. 
Hcgelmaier.—Zur Kenntniss einiger Lycopodien. Bot. Ztg, 1874. 
Farmer.—On Isoetes lacustris. Ann. Bot., Vol. V., 1891. 
Smith, R. Wilson.—The Structure and Development of the 
Sporophylls and Sporangia of Isoetes. Bot. Gaz., Vol. 
XXIX., 1900. 
Scott and Hill.—The Structure of Isoetes Hystrix. Ann. Bot., 
Vol. XIV., 1900. 
Isoetes echinospora, I Engelmanni, and I. lacustris. 
Stem. The cambium arises comparatively early, about the year 
after germination in the case of I. lacustris, in the parenchymatous 
tissue surrounding the axial strand. The divisions extend around 
and above the central cylinder, but not so far upwards as the 
youngest leaf-trace. The products of the cambial activity are 
parenchyma towards the periphery and elements of the so-called 
prismatic zone towards the centre. 
The amount of secondary xylem produced is always very 
small, and sometimes may be quite absent, a fact not surprising in 
in these aquatic plants. 
Isoetes Hystrix. 
Stem. As is well known the cambium in this and the other 
species examined is a well-marked tissue and arises very early in 
the life of the plant. There is a certain amount of variation in its 
position. In some cases it is situated in the tissue immediately 
towards the exterior of the primary xylem and between the wood 
and the phloem, in which case it may form new wood, the elements 
of which directly abut on the tracheides of the primary xylem. 
This activity however does not last very long, for a new merismatic 
zone arises further out towards the periphery of the stem. This 
new cambium forms secondary parenchyma towards the exterior 
while, on its inner side, it gives rise to parenchyma, phloem and a 
variable amount of xylem. The anomaly to he observed here is the 
formation of centrifugal phloem. 
This succession of cambial zones is not of general occurrence ; 
in the majority of instances the merismatic zone last considered is 
the only one produced, and it remains active throughout the whole 
period of secondary growth in thickness. 
