4i8 Scott and Hill.—Structure of Isoetes Hystrix. 
a single apical cell, and even described its mode of division in 
considerable detail. At that time,however, the single apical cell 
was the only type of growing-point which had been investigated, 
and it was natural that the great morphologist should have 
endeavoured to refer the stem, at least of any Vascular Crypto¬ 
gam, to that scheme. Later investigators have generally 
rejected his explanation. Hegelmaier (1874, p. 497) came to 
the conclusion that the meristem of the growing-point in 
/. velata and Dnriaei , at which he worked, had the signi¬ 
ficance of an apical cell-surface : he appears to regard this 
initial layer as extending over the whole area of the apex. 
Bruchmann (1874, p. 570) found in the case of /. lacustris , 
a definite initial group, occupying the middle of the apex. 
The apex of the stem, however, as shown in his figures 
(Plate XXIV, Figs. 29-31), bears a suspicious resemblance to 
a young leaf. Farmer, on the other hand (1890, Plate V, 
Figs. 13 and 14), working with serial sections, no doubt 
observed the true apex of the stem, which is comparatively 
flat. He finds that ‘ the entire apex of the stem is covered by 
a columnar layer of cells, which divide chiefly anticlinally, 
periclinal divisions only occurring at long intervals * (p. 39). 
He thus practically returns to Hegelmaier’s conclusion, and 
makes no attempt to identify a definite apical group. Van 
Tieghem (1891, p. 1429) states that the stem grows 4 like that 
of the Lycopods, by a single small mother-cell.’ We are not 
aware on what grounds this statement is made. Our own 
observations, while by no means decisive as to the mode of 
growth, show that Hofmeister’s opinion is at least defensible. 
In good transverse sections through the actual apex, we several 
times detected a large cell, or two large cells, in a central 
position. Fig. 1 is from a section showing the surface of the 
growing-point. A pair of large cells, with larger nuclei than 
their neighbours, is conspicuous at the centre of the apical 
region. The wall between them is delicate, which favours the 
idea that the larger of the two may be the actual apical cell, 
while its slightly smaller companion may be a segment just 
cut off from it. This was from one of the larger stems. 
