FILICALES 



347 



prothallus rots away. It is then only a matter of time and opportunity 

 for it to attain characters similar to those of the parent Fern-Plant. 



These are the salient features in the life-cycle of a Fern as it is seen 

 in its simplest form. They may be represented graphically to the 

 eye in a diagram (Fig. 291). The two 

 most notable points are those where the 

 individual is represented only by a single 

 cell, viz. the spore, and the zygote. These 

 are two landmarks between which inter- 

 vene two more extensive developments, 

 on the one hand the sexual generation or 

 prothallus, on the other the spore-bearing 

 generation, or Fern -Plant. If the events 

 above detailed recur in regular succession 

 the two phases of life will alternate. Of 

 these the one bears sexual organs, con- 

 taining sexual cells or gametes, and it 

 may accordingly be called the gameto- 

 phyte ; the other is non-sexual, but bears 

 sporangia containing the spores, and is 

 accordingly called the sporophyte. The 

 study of Ferns, and of Pteridophytes at 

 large, leads to the conclusion that this regular alternation is typical for 

 them all. These two alternating generations differ not only in form but 

 also in their relation to external circumstances, and especially in the 

 water-relation. The sporophyte is structurally a land growing plant, 

 with nutritive, mechanical, and conducting tissues, and a ventilating 

 system. Not only is it capable of undergoing free exposure to the 



FIG. 289. 



Adiantum Capillus Veneris. The 

 prothallus, pp, seen from below has a 

 young Fern-plant attached to it. b= 

 first leaf, w, te'= first and second 

 roots, h = root hairs of the prothallus. 

 ( x about 30.) (After Sachs.) 



FIG. 290. 



Adiantum Capillus Veneris. Longitudinal section^through the prothallus, p, and 

 young Fern-plant E. h = root hairs of prothallus. =archegoma. b= the first leaf. 

 w = the first root of the embryo. ( x 10.) (After Sachs.) 



ordinary atmospheric conditions, but dryness of the air is essential 

 for the final end of its existence, viz. the distribution of its spores. 

 On the other hand, the gametophyte is structurally a plant ill-fitted 

 for exposure, with undifferentiated and ill-protected tissues and no 



