of Pilularia globulifera , Z. 243 
burger \ Arcangeli 2 , and others, the wall of the spore is very 
complex. 
On the outside is a mucilaginous layer that swells up greatly 
when placed in water. Below this is a very characteristic 
layer, showing a prismatic structure (PL XIII, Fig. 23 a). It is 
to the sudden thickening of this layer that the enlargement of 
the upper part of the spore is due. Within this two layers 
(Fig. 23. b, c ) are further to be distinguished before the 
endosporium (d) is reached. Of these the outer is thicker 
and presents a punctated appearance. Sections stained with 
safranin, or gentian-violet, show the structure of the wall of 
the spore very plainly. 
Owing to the mucilaginous character of the outer coat, as 
well as the delicate character of the spore- contents, it 
is quite impossible to get satisfactory sections of the fresh 
spore, and recourse must be had to fixing agents. Various 
ones were used with good results, but on the whole absolute 
alcohol, in which the spores should be left for two or three 
days at least, was found the best. A 1 per cent, chromic 
acid mixture, and Flemming’s mixture of chromic, acetic and 
osmic acids were also successfully used, but care must be taken 
to thoroughly wash out the acids before further treatment. 
In making the sections the spores were imbedded in paraffin, 
and then cut with a Cambridge rocking microtome. Schon- 
land’s methods 3 , with some simplifications, were used in most 
cases, but in others the spores were gradually brought into 
clove oil, and then into xylol instead of turpentine. This 
method requires little time, and often gives excellent results, 
but is not always to be relied on, though in the early stages 
it answered very well, and the penetration of the paraffin 
was facilitated. When chromic acid mixtures were used, the 
specimens were brought gradually into absolute alcohol, which 
was then replaced by clove oil, and finally by a saturated cold 
solution of paraffin in turpentine before being placed in the 
1 Bau u. Wachsthum der Zellhaute. 
2 1. c., p. 323. 
3 Bot. Centralblatt, 1887, No. 22. 
