mw^i 



THE BRYOPHYTA: MUSCI, THE MOSSES 419 



one or two cells, and similar teeth are also scattered along the back of the 

 midrib. In section the midrib is seen to project more on the under than 

 on the upper surface of the leaf, and in it towards the lower side there are a 



number of thin-walled cells continuous with 

 the conducting cells of the foliar strands in 

 the cortex (Fig. 402). 



Although Mnium is provided with abund- 

 ant rhizoids and with a central strand of 

 elongated cells in the stem, it has been 

 shown by experiment that a considerable 

 amount of w^ater is carried up by capillarity 

 on the surface of the plant and absorbed 

 directly by the thin-walled cells at the apex 



Fig. 401. — Mniiun Iioniu>n. 

 Leaf showing midrib and 

 paired teeth on thickened 

 margin. 



Fig. 402. — Mniiuii Iwrnum. Trans- 

 verse section of midrib of leaf 

 showing strand of thin-walled 

 hydroids surrounded by thickened 

 fibre cells. {After Muller-Berol.) 



and at the leaf bases. In many Mosses the amount so absorbed greatly 

 exceeds the amount carried up internally. Mnium is an exception in this 

 respect in that more water ascends through the axial cylinder than by the 

 surface, and this may be due to the fact that its leaves are more widely 

 spaced than is the general rule. 



Sexual Reproduction 



As we have already said, the sex organs are borne on different plants, en- 

 closed in a rosette of leaves termed the involucre. The head of the sex organs 

 is sometimes called a gametoecium, and the leafy shoot the gametophore. 

 Towards the centre of the gametoecium the leaves become progressively 

 smaller, and in their axils, and particularly the axils of the inner ones, the sex 

 organs develop, together with special filamentous structures termed 

 paraphyses. Beneath these reproductive heads the stem broadens out, 

 forming a flat receptacle, and in this region the central conducting tissue 

 also broadens out and ends in a disc. 



