500 Mr. Valentine's Supplementary Observations on 



theca (Tab. XXXV. fig. 8.), we shall find that the space between the thecal 

 and sporulai" 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 (Tab. 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 he understood as 

 generally applicable, for there are many species in which the columella is crowded with green gra- 

 nules. 



