The Development of the Spores of Equisetum. 265 
tapetum occurs, the special-wall is seldom found wanting and is 
usually well developed. 
Lycopodium, Isoetes and Ipomcea are all rather striking examples 
in which a secretion-tapetum is accompanied by massive special- 
walls. 
I believe that the explanation of this relation is to be found in 
the necessity for providing protection for the young spores in those 
cases in which a secretion-tapetum exists. The soft mucilaginous 
special-walls, which usually form a more or less massive investment 
to the spore-elements in sporangia provided with secretion-tapeta, 
serve as a moist packing round and between the spores, shielding 
these bodies from mechanical shocks and jars, and, above all, 
protecting them from too great a loss of water. Moreover, whilst 
they carry out this protective function, the special-walls are 
sufficiently extensible to permit of the growth of the spore- 
protoplast, and they do not interfere with the passage of the 
nutrient materials required by the developing spores. 
In those sporangia in which a tapetal-plasmodium is formed at 
an early stage in development, this protoplasmic investment protects 
the young spores from mechanical injuries and dessication suffi¬ 
ciently to render the additional formation of a massive special-wall 
unnecessary. 
The young spores of the tetrad separate from one another and 
lie, still without any cell-wall round them, completely enveloped in 
the tapetal plasma. 
After a short time a cell-wall is developed round the spore- 
protoplast, and later a delicate cell-membrane is seen to line the 
tapetal vacuole in which the young spore lies. 
The future history of these walls shows that the membrane 
round the spore is the exospore, whilst the membrane formed by the 
tapetum is the “ middle layer.” 
I differ from my predecessors, therefore, in ascribing the 
“ middle layer” of these spore-walls to the secretory activity of the 
tapetal plasma. I can find absolutely no evidence to support the 
view that a superficial lamellum of the exospore is split off to form 
a “ middle layer,” or that this layer has any relation to the spore- 
protoplast. Neither can it be derived from the transformation of the 
special-walls, as some botanists have supposed, since this membrane 
does not occur in Equisetum. Following the subsequent history of 
these spore-walls we find that the tapetal activity produces another 
layer of cell-wall which is formed on the outside of the “ middle 
