214 - Multicellular Plants 



the stalk and spore capsule may contain some 

 chloroplasts and perform some photosynthe- 

 sis; but the sporophyte obtains most of its 

 nourishment from the gametophyte by ab- 

 sorption through the foot. 



At maturity many spore mother cells, or 

 sporocytes, are formed in the sporangium. 

 Each of these diploid cells eventually pro- 

 duces four spores, by meiosis; and later, when 

 the lid of the sporangium (Fig- 12-12) 

 drops off, a multitude of tiny haploiil spores 

 are discharged into the dry atmosphere. 



As in other higher plants, the spore rep- 

 resents the beginning of the gametophyte 

 generation. In the mosses, however, more 

 than one gametophyte may arise from one 

 spore, if it happens to fall in a favorable 

 locality. When the spore germinates, it bursts 

 its protective cover and begins to divide rap- 

 idly by mitosis (Fig. 12-12). Initially this 

 produces a branching filament that closely 

 resembles some of the green algae, from 

 which the mosses presumably have arisen. In 

 the moss this young gametophyte is called a 

 protonema; and each protonema may give 

 rise to several erect leafy gametophytes by a 

 process of budding (Fig. 12-12). Finally 

 the protonema dies off, isolating the several 

 erect leafy gametophytes, which soon mature 

 and produce the gametes of the next gener- 

 ation. 



SUMMARY: THE LIFE CYCLES OF THE 

 MOSS AND FERN 



In summary, the life cycle of the mosses 

 displays a clear general resemblance to the 

 cycle of the ferns. In both cases: (1) the hap- 

 loid sexual gametophyte alternates regularly 

 with the diploid asexual sporophyte; (2) the 

 haploid spores are resistant to dryness, and 

 are well adapted for dissemination through 

 the atmosphere; and (3) the sperm require 

 an abundance of water in order to reach the 

 eggs. However, the gametophyte, which is 

 dominant in the Bryophyta, is relatively in- 

 conspicuous in the Filiceneae, although the 

 fern gametophyte still carries on an inde- 



pendent nutrition. The sporophyte, on the 

 other hand, which is relatively inconspicu- 

 ous and dependent in Bryophyta, is the dom- 

 inant independent generation in Filiceneae. 

 These evolutionary tendencies continue, 

 reaching a maximum in the seed plants. 

 Among seed plants, the gametophyte gener- 

 ation is further reduced, to microscopic di- 

 mensions, and the gametophyte is completely 

 dependent on the large dominant sporo- 

 phyte. This development has enabled the 

 seed plants to spread to much drier regions 

 of the earth, since the sperm are free of the 

 necessity of swimming through water on 

 their journey toward the eggs (see later). 



LIFE CYCLE OF SELAGINELLA 



One of the club mosses (p. 613), Selaginella 

 and a few related forms show transitional 

 developments which foreshadow a further 

 reduction of the gametophyte, such as oc- 

 curs in all seed plants. The sporophyte of 

 Selaginella (Fig. 12-13) has a profusely 

 branched creeping stem, with short erect 

 branches and small scalelike green leaves. 

 The sporangia are borne in conelike struc- 

 tures at the tips of the erect branches (Fig. 

 12-13). But Selaginella has two kinds of spo- 

 rangia: (1) microsporangia, in which the 

 spores (microspores) are relatively small; 

 and (2) macrosporangia, in which the spores 

 are about a hundred times larger (Fig. 

 12-13). 



When shed, each microspore develops into 

 a very small (microscopic) male gametophyte, 

 which lacks chlorophyll, and consists mainly 

 of a single antheridium (Fig. 12-13). This 

 growth occurs inside the protective cover of 

 the microspore, at the expense of organic 

 substances already present in the spore at 

 the time of shedding. Meanwhile, the macro- 

 spore develops into a female gametophyte, 

 which also lies mainly inside the old spore 

 casing (Fig. 12-13). When mature, the female 

 gametophyte displays several poorly differ- 

 entiated archegonia, each containing a single 

 egg cell. The female gametophyte has very 



