MOSSES 



197 



366. Section through a receptacle of 

 Polytrichum commune, showing 

 paraphyses and antheridia. 



exposing the chlorophyll cells to the air, but in dry weather the mar- 

 gins of the leaves roll inward, and the leaves fold closely against the 

 stem, thus protecting the delicate assimilating tissue. 



The antheridia and archegonia of polytrichum are borne in groups 

 at the ends of the branches on different 

 plants (many mosses bear both organs 

 on the same branch). They are sur- 

 rounded by involucres of characteristic 

 leaves termed perichaetia or perichxtal 

 leaves. Multicellular hairs known as 

 paraphyses are scattered among the 

 archegonia and antheridia. The invo- 

 lucres with the organs borne within them are called receptacles or, less 

 appropriately, "moss flowers." As in marchantia, the organs are very 

 minute and must be highly magnified to be studied. 



The antheridia are borne in broad cup-like receptacles on the 

 antheridial plants. (Fig. 366.) They are much like 

 the antheridia of marchantia, but they stand free 

 among the paraphyses and are not sunk in cavities. 

 At maturity they burst and allow the sperm-cells 

 or spermatozoids to escape. 

 In polytrichum when the re- 

 ceptacles have fulfilled their 

 function the stem continues 

 to grow from the center of the 

 cup. (Fig. 367, m.) The arch- 

 egonia are borne in other re- 

 ceptacles on different plants. 

 They are like the archegonia 

 of marchantia except that they 

 stand erect on the end of the 

 branch. 



The sporogonium which de- 

 velops from the fertilized egg 

 is shown in Fig. 367, a, b. It 

 consists of a long, brown stalk 

 bearing the spore-case at its 

 summit. The base of the stalk 



is embedded in the end of the 367 ' Polytrichum commune;/,/, fertile plants, 



one on the left in fruit ; m, antheridial plant. 



moss stem by which it is nour- 

 ished. The capsule is entirely inclosed by a hairy cap, the calyptra, b. 

 The calyptra is really the remnant of the archegonium, which for a time 



