54 Farmer . — On Isoetes lacustris, L. 
of Isoetes is not perhaps, strictly speaking, a primary structure, 
and I incline rather to consider it as an early type of adven- 
titious root. It is formed relatively much later than in the 
true Ferns, and yet it is far more exogenous in character than 
in them. 
The Oophyte . — The development of the oophyte has long 
presented difficulties to those who have attempted its investi- 
gation, partly owing to the ease with which the contents of the 
spore are injured, and partly to the hardness and thickness of 
its coats. 
Mettenius 1 , who described the structure of the macrospore, 
correctly distinguished its three principal layers or coats. 
The outer glassy epispore or perinium is extremely hard and 
brittle, and its surface is beset with numerous prominences. 
It is without doubt derived from the plasma of the sporangium 
formed by the breaking down and solution of the tapetal cells 
(and probably the mother-cell walls), and it stains faintly with 
safranin. The exospore, a dark brown band of somewhat 
fibrous appearance, is clearly resolvable into a double layer, 
and the outer one of the two frequently splits again. The 
endospore is a rather thick cellulose-wall, of irregular thick- 
ness, and it is characterised by a brightly refractive appearance, 
recalling that presented by collenchymatous walls. 
The contents of the spore are rendered obscure by the enor- 
mous quantity of oil, the removal of which can however be 
easily effected by turpentine or ether, when the protoplasm 
appears as a granular and reticulated structure. The granu- 
larity is caused by the numerous small proteid bodies which 
are embedded in its substance, whilst the reticular structure is 
due to the withdrawal of the oil which formerly filled the 
meshes. Starch-grains are also freely present, so that the 
protoplasm is thus very richly supplied with nutritive reserve 
material. At the upper end of the spore lies the nucleus, 
which is of very large dimensions, and contains a variable 
number of nucleoli of different sizes. These points are illus- 
trated by the photographs (Figs. 24, 25). The nucleus, which 
1 Mettenius, Ueber Azol/a, Linnaea, 1847, p. 269. 
