790 
Notes. 
The guard-cells, which are smaller than the other epidermal cells, 
lie flush with the surface of the cotyledon ; they contain large, deeply 
staining nuclei. In surface view they appear crescent-shaped, enclosing 
a small, round or slightly oval pore .in the centre (Fig. 29 A). 
In transverse section it is seen that the pore opens into a small 
intercellular space which in some cases appears to be filled with loose- 
celled tissue. 
The guard-cells are oval in transverse section and obliquely inclined 
towards one another at the surface, their walls are thicker than those 
of the neighbouring cells but do not seem to be cuticularized. 
In Cycas revoluta the cotyledons are much thicker and narrower 
than those of Ginkgo ; they are more closely connected with the 
surrounding endosperm and their inner flattened surfaces are fused 
together, so that the junction of the two can only be distinguished near 
the margin, where the epidermal layers are still marked out, and by the 
thickening of the cell-walls of a few smaller cells here and there in the 
central part. 
In spite of this very considerable alteration which the cotyledons 
have undergone stomata are still recognizable in a transverse section 
of the cotyledon. 
The guard-cells here are much smaller than the neighbouring cells, 
and of characteristic shape. They contain large, deeply staining 
nuclei, and their walls are very much cuticularized on what would be 
their outer surface if the cotyledons were not fused. A well-marked 
pore leads into an intercellular space below (Fig. 29 B). 
It is evident from the position of the stomata in Cycas revoluta that 
they cannot possess any functional value ; probably they only indicate 
an ancestral condition when the cotyledons came above ground and 
functioned as ordinary foliage leaves. 
In Ginkgo , however, where the cotyledons are not fused, there would 
be a layer of air between the two cotyledons, between which there 
often exists a considerable space, and here they might possibly have 
a respiratory function. 
At all events, whether functional or not, their presence suggests that 
at one time the cotyledons of Ginkgo were withdrawn from the testa 
and expanded above ground, as is the case in most Gymnospermous 
seedlings. 
There seems, therefore, good reason to regard the cotyledons of 
Cycadaceae and Ginkgoaceae as true foliage leaves, which have 
