THE VASCULAR CRYPTOGAMS 43 



the clusters of sporangia, or sori, as they are called, are 

 very conspicuous on the under-surface. They are 

 usually absent from the basal part of the leaf. On the 

 larger segments, the sori are arranged in two short 

 rows, one on each side of the midrib (see Fig. 18, B), 

 while on the smaller segments there may be only 

 one or two sori altogether. Each sorus is covered 

 by a kidney-shaped membranous envelope called the 

 indusium, and is seated just over one of the lateral veins. 

 The individual sporangia, which cannot be distinguished 

 without the aid of a lens, are very numerous in each 

 sorus, and every sporangium contains a large number 

 of spores, so that the reproduction of the plant is 

 extremely well provided for. 



On germination, each spore gives rise to a protliallus, 

 which is a much larger structure here than in Selaginella, 

 and leads quite an independent existence (see Fig. 31, 

 p. 63). The prothallus is a flat, green, heart-shaped 

 body, sometimes as much as half an inch in diameter, 

 attached to the soil by the root - hairs which arise 

 from its under - surface. Prothalli may be found in 

 abundance covering the damp ground where Ferns are 

 growing. In Ferns the same prothallus usually 

 bears both kinds of sexual organs, the antheridia and 

 archegonia. After fertilisation the ovum formed in one 

 of the archegonia becomes an embryo, which eventually 

 grows up to be a new Fern plant. 



In Ferns, then, we have, in normal cases, a sharp 

 alternation of generations. The Fern plant is the asexual 

 generation, or sporophyte, producing the sporangia, and 

 ultimately the spores. The prothallus is the sexual 

 generation, or oophyte, producing the antheridia and 

 archegonia, in which the sexual cells are developed. 



