SPHAGNUM. 17 



while accurate observation at the points where the longitudinal 

 walls have been cut through will show that these circular mark- 

 ings are actually open pores, by means of which the cavities of 

 the cells of this tissue communicate one with another, and are 

 ultimately in open communication with the medium in which the 

 plant grows. This tissue serves as a capillary system, by means 

 of which water is supplied to the inner tissues of the stem. 



XX. Strip off one or two leaves : mount in water, or very weak 

 glycerine, and examine under a low power: note 



1. The simple form of the leaf, and its entire margin. 



2. The absence of any midrib. 



3. The simple structure, the leaf consisting only of a single 

 layer of cells, amongst which two types may be recognised under 

 a high power, viz. 



a. Large colourless cells, the walls of which are marked by 

 annular or spiral bands, and showing here and there round open 

 pores similar to those already observed in the superficial layers 

 of the stem: these cells have no cell-contents. 



b. Smaller cells of narrow form, easily recognised by their 

 containing green chlorophyll granules embedded in colourless 

 protoplasm : these cells, being attached by their ends one to 

 another, together form a network, the meshes of which are 

 occupied by the large colourless cells, a. 



XXI. It may be found difficult to obtain material for the 

 practical study of the sexual organs of Sphagnum, and it will 

 accordingly suffice here to refer to the descriptions given of them 

 in text-books (e.g. Goebel, Outlines of Classification, p. 183, &c.). 



If specimens of the sporogonia be at hand, note with the 

 naked eye 



1. The spherical form of the sporogonium. 



2. Its position, seated on the end of a more or less elongated 

 stalk the pseudopodium : this must not be confounded with 

 the seta of other Mosses ; as sections will show, the true seta in 

 Sphagnum is short, and the pseudopodium is merely a prolongation 

 of the axis of the oophore generation. 



3. The calyptra, which may be, according to age, a more or 

 less complete covering of the sporogonium, and is derived from 

 the wall of the archegonium. 



4. Where the calyptra has been broken away or removed, the 

 circular lid-like operculum may be seen at the apex. 



C 



