INTRODUCTORY TO LAND-VEGETATION 



457 



by reduction the original number of the chromosomes is restored. 

 The cycle may be represented diagrammatically by Fig. 351, and 

 the two phases, the haploid (" X ") and the diploid (" 2X ") constitute 

 the alternating " generations.'' They appear as the gametophyte and 

 the sporophyte respectively. These " generations " may bear varying 

 proportions one to the other, that which is the more prominent and 

 obvious being referred to in popular phraseology as " the plant." But 

 it is, in fact, only one phase of the complete life history. For instance, 

 in the Bryophytes it is the X -generation or gametophyte which is the more 

 conspicuous, and in a Moss the leafy body which is called the " Moss- 

 Plant " turns out to be the gametophyte and bears the sexual organs, 

 while the sporophyte is the capsule dependent upon it (see Fig. 352). 

 This is so for all of the Bryophytes. In 

 the Pteridophytes, however, it is the 2X- 

 generation or sporophyte which is pre- 

 dominant, and the leafy structure known 

 as the " Fern-Plant " is the sporophyte 

 generation, while the gametophyte is the 

 relatively small prothallus which pro- 

 duces it (Figs. 353, 354). This is so for 

 all of the Pteridophytes. When it is 

 remembered that in Dictyota and Poly- 

 siphonia the two generations are very 

 much alike, this difference of balance 

 will seem less strange than it might 

 otherwise appear to be. For the pro- 

 gression to a leafy shoot may in fact 

 have been carried out in either of them. 

 Most of the Archegoniatae possess 

 a shoot composed of axis and leaves, a 

 type of development of the plant-body which is continued in Seed- 

 Plants, and is evidently suited, by its relatively large proportion of 

 surface to bulk, to the conditions of subaerial life. Exposure to the 

 air of organs charged as they are with water leads to transpiration. 

 Their ventilation-system of intercellular spaces opening at the 

 numerous stomata facilitates this. The loss is made good from an 

 absorptive system in the soil, which is itself nourished by the activity 

 of photosynthesis in the shoot. This implies both water-conduction 

 and transit of plastic materials by means of a vascular system, such as is 

 found in all of the larger forms of land-living plants, in which the 

 •• plant " is in all cases the sporophyte generation (Fig. 257, p. 335). 



Fig. 353. 



Adiantum Capillus Veneris. The 

 prothallus, pp, seen from below has a 

 young Fern-plant attached to it. b = 

 first leaf. w, w' = first and second 

 roots, h = root hairs of the prothallus. 

 ( x about 30.) (After Sachs.) 



