Reproduction in Multicellular Plants - 205 



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Fig. 12-3. Magnified vertical section of a fern leaf, 

 passing through a sorus. Three of the clustered spo- 

 rangia show quite clearly; and the ripening spores 

 can be seen through the side walls of each sporan- 

 gium. (From The Plant World, by Fuller and Carothers. 

 Holt, Rinehart and Winston, Inc.) 



The prothallia grow in moist shaded places 

 on the ground, or on decaying logs; and 

 these plants are not recognized as "ferns," 

 except by those who have traced out their 

 origin (Fig. 12-5). 



The prothallium of a fern usually meas- 

 ures less than half an inch at the widest part. 

 Nevertheless the prothallium grows inde- 

 pendently like other green plants. All the 

 cells of the prothallial body possess chloro- 

 plasts; but the numerous rhizoids, which 

 grow down into the soil from the underside 

 of the prothallium, are colorless. During 

 growth, the rhizoids absorb water and min- 

 eral salts for the whole prothallium; and sev- 

 eral weeks after a spore germinates, the pro- 

 thallium is sexually mature. 



When mature, the gametophyte of the fern 

 produces gametes. Both male and female or- 

 gans develop on the undersurface of each 

 prothallium. Usually the egg-forming organs, 

 called archegonia, lie near the indented mar- 

 gin (apical notch) of the prothallium (Fig. 

 12-4), and each archegonium contains a sin- 

 gle egg. The egg lies in a hollow chamber, 

 the venter, which communicates with the en- 

 vironment through a short tubular channel, 

 the neck. The sperm-forming organs, called 



antheridia, are usually situated among the 

 rhizoids, nearer the other end and margins 

 of the prothallium (Fig. 12-4). Each anther- 

 idium is a hollow structure from which a 

 number of delicate flagellated sperm are fin- 

 ally liberated (Fig. 12-4). 



The sperm are usually liberated in rainy 

 weather, and they must swim through water, 

 underneath the prothallium, in order to 

 reach the archegonia. However, the arche- 

 gonia and antheridia of any one prothallium 

 do not reach maturity at the same time, and 

 consequently the sperm that fertilize the 

 eggs are derived from another nearby pro- 

 thallium. In many cases, the archegonia pro- 

 duce secretions that attract the sperm to- 

 ward the eggs; frequently a swarm of sperm 

 will enter the neck of a single archegonium. 

 However, only one sperm normally succeeds 

 in penetrating the egg. 



The fertilized egg marks the beginning of 

 a new sporophyte generation. This single 

 diploid cell, while it still lies in the arche- 

 gonium of the parent gametophyte, divides 

 repeatedly by mitosis, and gives rise eventu- 

 ally to all the cells of the new sporophyte 

 (Fig. 12-6). During the early stages of this 

 growth, the young sporophyte depends upon 

 the parent gametophyte for its organic nutri- 

 ents (Fig. 12-6). But soon the growing sporo- 

 phyte develops its own root, stem, and leaf 

 systems (Fig. 12-4), and thereafter the sporo- 

 phyte carries on an independent holophytic 

 nutrition. In about one year, the sporophyte 

 reaches maturity and produces a new crop of 

 spores — which completes the life cycle of the 

 species. 



HAPLOiDY AND DIPLOIDY IN HIGHER 

 PLANTS 



Generally speaking the foregoing life cycle 

 is typical of all higher plants — as is shown 

 diagrammatically in Figure 12-7. In all higher 

 plants the diploid zygote marks the begin- 

 ning of the sporophyte generation; and since 

 the cells of the sporophyte are all derived by 

 mitosis from this diploid cell, all the cells of 



