200 



BEGINNERS' BOTANY 



dry weather the margins of the leaves roll inward, and the leaves 

 fold closely against the stem, thus protecting the delicate assimi- 

 lating tissue. 



The antheridia and archcgonia 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 pcricha-tia 

 o\ pcrichaital leaves. Multicellular hairs known as /<?;v7///v^vj' are 

 scattered among the archegonia and antheridia. The involucres 

 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. 297). 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 poly- 

 trichum, when the receptacles have 

 fulfilled their function, the stem con- 

 tinues to grow from the centre of 

 the cup (w. Fig. 295). The arche- 

 gonia are borne in other receptacles 

 on different plants. They are like 

 the archegonia of marchantia except that they stand erect on the 

 end of the branch. 



The sporogonuim which develops from the fertilized egg is 

 shown in a, h, Fig. 295. It consists of a long, brown stalk bearing 

 the spore-case at its summit. The base of the stalk is imbedded 

 in the end of the moss stem by which it is nourished. 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, increases in size to accommodate and protect the young 

 growing capsule. It is finally lorn loose and carried up on the 

 spore-case. The mouth of the capsule is closed by a circular lid, 

 the operculum, having a conical projection at the centre. 



The operculum soon drops, or it may be removed, displaying a 

 fringe of sixty-four teeth guarding the mouth of the capsule. This 

 ring of teeth is known as the peristome. In most mosses the 

 teeth exhibit peculiar hygroscopic movements; i.e. when moist 

 they bend outwards, and upon drying curve in toward the mouth 

 of the capsule. This motion, it will be seen, serves to disperse 

 these spores gradually over .1 lorjj period of time. 



Not Wvi entire capsule is filled A^dth spores. There are no 

 elaters, but the centre of the capsule is occupied by a columnar 



Fig. 297. — Section through a 

 Receptacle of Polytri- 

 chum COMMUNE, showing 

 paraphyses and antheridia. 



