FILKALKS 



505 



spore, and the zygote. These are two landmarks between which 

 intervene 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, containing sexual cells or gametes, and 

 it may accordingly be called the gametophyte ; 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 



Fig. 398. 

 Adiantum Capillus Veneris. 



The 



prothallus, pp, seen from below has 

 a young Fern-plant attached to it. 

 6 = first leaf, w, ifl'=first and 

 second roots, h = root hairs of the 

 prothallus. ( x about 30.) (After 

 Sachs.) 



Fig. 399. 

 Adiantum Capillus Veneris. Longitudinal section 

 through the prothallus, pp, and young Fern-plant £. 

 fc = root hairs of prothallus. a = archegonia. 6 = the 

 first leaf. u> = the first root of the embryo. ( x 10.) 

 (After Sachs.) 



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 under- 

 going free exposure to the 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 gameto- 

 phyte is structurally a plant ill-fitted for exposure, with undiffer- 

 entiated and ill-protected tissues and no ventilating system, while 

 the object of its existence, viz. fertilisation, can only be secured 

 in the presence of external liquid water. As regards the water-relation 



