The Development of the Spores of Equisetuin. 261 
THE DEVELOPMENT OF THE SPORES OF 
EQUISETUM. 
f 1HE structure and the development of the spores of Equisetuin 
were very favourite subjects for study with the older botanists. 
Sachs, Sanio, Hofmeister, Russow, Tschistiakoff, and Leitgeb have 
all contributed to this subject, whilst of living botanists none has 
given us so valuable an account as Strasburger. 
All are now agreed that the mature spore of Equisetuin 
possesses four layers to its wall. On the extreme outside there is 
the spirally cleft wall which constitutes that curiously hygroscopic 
structure—the elater. Within this layer is a membrane which is 
usually spoken of as the “middle layer”; within this lies the 
“exospore,” 1 whilst the innermost and last-formed layer of all is 
the endospore. 
When, however, we turn to the literature for information 
regarding the origin and interpretation of these four layers of the 
spore-coat we find a surprising diversity of opinion. 
Without describing the opinions and labours of the different 
investigators in detail, I may briefly summarise the views which 
have been expressed regarding the development of the wall of these 
spores in the following manner : — 
1. All four layers are formed by the spore-protoplast in 
centripetal succession. 
2. The outer (forming the elater) and middle layers are derived 
from the special-mother-cell wall, the exospore and endospore from 
the spore-protoplast. 
3. The outer layer is derived from the special-mother-cell wall, 
the middle layer, the exospore and the endospore are formed by the 
spore-protoplast 
4. The outer layer is formed by the tapetal cytoplasm, the 
middle layer, the exospore and the endospore are formed by the 
spore-protoplast. 
I have recently re-examined the development of the spores of 
two species of Equisetuin ( E. arvense and E. lit no sum ) with the 
principal object of attempting to decide which of the above accounts 
is the correct one. 
1 According to the interpretation which is given to the “ middle 
layer” the “ exospore” mentioned above corresponds to the 
whole or only to a part of the true exosporc. As will be seen 
below I regard it as the true exospore in its entirety. 
