500 Mr. VALENTINE's Supplementary Observations on 
theca (Tas. XXXV. fig. 8.), we shall find that the space between the thecal 
and sporular membranes communicates with the loose tissue in connexion 
with the stomata, so that the air has free access to the opposing surfaces of 
those membranes; and it is a fact, that all the tissue thus exposed to the 
action of the atmosphere contains green granules, whilst in those parts of the 
tissue, the columella for instance, which may be supposed to be inaccessible to 
the air, the granules are smaller and colourless*. ‘The proof is then satisfac- 
tory, that the last-described layer is the outer surface of the sporular mem- 
brane, which consists of certainly not more than two layers of cells; for I 
am confident, from the great number of species which I have examined in all 
stages of their growth, that this thickness of the sporular membrane is never 
exceeded. I have, in many species, had a difficulty in deciding whether there 
were more than one layer, but never that there were not more than two. It 
follows, then, that the next layer, after counting two for the sporular mem- 
brane, will either be the columella or a tissue occupying the sporular cavity. 
That this last is the truth, is proved by the fact of the succeeding layer being 
composed of decided sporules. The cells forming the layer which intervenes 
between the sporules and the sporular membrane are tinged with yellow, in 
which respect they resemble the sporules before they have arrived at ma- 
turity; and between the columella, which is composed of colourless cellules, 
and the layer of sporules, are two rows of cells, also coloured yellow similar 
to the last, and also occupying the sporular cavity. In this stage the sporules 
already formed consist of a disjointed cell containing a quantity of grumous 
granular matter. In some of the cells the grains are uniformly scattered, and 
in others they begin to arrange themselves in groups around four grains of 
larger development (Tas. XXXV. fig. 10.), which ultimately become four di- 
stinct sporules contained in a mother-cell. It is perfectly certain that the 
single row of sporules which exists in this stage is not equivalent to the 
thousands of sporules which exist at the time of maturity, and it therefore 
follows that more must be formed. Now as it has been already proved that 
the yellow cellules occupy the sporular cavity, and as they are exactly similar 
* 'This remark, that the columella does not contain green granules, must not be understood as 
generally applicable, for there are many species in which the columella is crowded with green gra- 
nules. 
