572 Mr. Henperson on the Reproductive Organs of Equisetum. 
membranous bag, flattened on two sides, and reticulated in a peculiar man- 
ner; six or more thecae surround the pedicel of the peltate scale to which 
they are attached, the approximating sides being flattened, and the outer and 
inner edges prominent. The theca is reticulated in the following manner: 
three or more rows of muriform meshes placed transversely side by side run 
along the inner edge (Tas. XXXIX. fig. I.); the ends of these meshes are 
separated by a zig-zag line (or what proves when highly magnified to be a 
double line), and it is at one of these lines that the dehiscence of the theca 
takes place (Tan, XXXIX. fig. 14.a.). The meshes oceupying the sides and 
outer edge are of various forms, generally disposed in irregular rows or 
divided into areolæ, the meshes in each of which have a different inclination ; 
those on the outer edge are placed longitudinally, from which the bundles of 
meshes on the sides are variously inclined in the direction of, and to meet the 
rows on the inner edge (Tas. XXXIX. fig. 2.). In this state the theca con- 
tinues until the spores have attained their full size, and are changing to the 
dark green colour which they assume at maturity. 
At this time a new deposit of vegetable matter is added to the membrane of 
the theca, in which spiral vessels are developed: a deposit of green granular 
matter is first observed round the edges of the meshes, and afterwards di- 
spersed of a lighter colour over the whole: the development of the spiral 
vessels immediately follows this deposit, which seems in a great measure to 
be absorbed in the process, as the theca afterwards partly regains its former 
transparency. On first observing this deposit, it appeared to me to be formed 
on the inner surface of the membrane of the theca; but after examining both 
surfaces with a magnifying power of four hundred diameters, I found that they 
presented exactly the same appearance. Subsequent observations, however, 
have convinced me that the meshes of the theca are composed of a double 
membrane; that they are in fact flattened cells, the united edges of which 
form the boundaries of the meshes, and that the deposit in question, and 
afterwards the spiral vessels, are formed in the space caused by a partial 
separation of the two membranes. This observation is confirmed by the 
appearance of the meshes, which at this time have more or less of a blistered 
appearance, or are wholly inflated, but again collapse as soon as the spiral 
vessels are formed. These vessels vary according to the forni of the meshes 
