ALTERNATING GENERATIONS 



off against this, a remarkable power of recovery on the return of moisture 

 after being dried up. The sexual organs are usually borne by the Moss- 

 Plant at or near to the apex of its upward-growing branches. The result 

 of fertilisation here again carried out by spermatozoids motile through 

 water is the formation of the Moss-Fruit, or sporophyte, which is 

 throughout life a mere appendage on the Moss-Plant. At first it is, 

 like that of Riccia^ completely enclosed by the venter of the archegonium 

 (Fig. 20) ; but it soon shows apical growth and 

 elongation : the venter is then ruptured trans- 

 versely, and the sporogonium is exposed. As 

 it elongates its base remains embedded in the 

 tissue of the Moss-Plant : its apex is still 

 covered by the upper part of the archegonial 

 wall, the calyptra; but at ripeness this is shed, 

 and the enlarged capsule on dehiscence is able 

 freely to scatter its spores. After this the 

 ephemeral sporophyte dies away. 



Comparing such a Moss with Riccia, the 

 phases of the life-history correspond, but their 

 elaboration is different : the thalloid gametophyte 

 of Riccia is replaced by the upright leafy plant 

 of Catharinea. The fertilisation is still dependent 

 on fluid water, but its product is more complex : 

 there is in Catharinea a distinction of apex and 

 base, with localised apical growth ; but the form 

 is still relatively simple, the whole construction 

 being on the radial type, without appendages. 

 The spore-production is restricted to the upper 

 region, and takes place in one continuous sac. 

 The sporophyte is still borne and nourished 

 throughout its life by the parent gametophyte ; 

 but it is able by its chlorophyll-containing cells 

 to carry on photosynthesis in some degree, as 

 an accessory to the supply derived from the 

 parent. It has a ventilating system like that characteristic of aerial 

 plants, while this is absent from the gametophyte. 



A comparison of such a Moss with a Lycopod shows a different 

 balance of the two alternating generations. The gametophyte of Ly co- 

 podium cernuum is shown in Fig. 21 as a somewhat massive structure, 

 bearing lobes of chlorophyll-containing tissue above, which have sometimes 

 been compared with the leaves of a Bryophyte : below it is attached by 

 rhizoids to the soil. Like the Moss it is an independent organism capable 

 of self-nourishment. It bears its sexual organs about the bases of the 

 lobes, and is dependent upon external fluid water for its fertilisation. 

 Notwithstanding its massive bulk it is without a ventilating system. The 



FIG. 20. 



Young sporogonium of Physco- 

 mitrella patens, Br. and Sch., 

 shortly before the rupture of the 

 archegonial wall. (After Hy.) 



