IN LYCOPODS 



39 



the soil, and capable of complete self-nourishment for an extended 

 period before the production of spores. Moreover, these are produced, 

 not in a single sac, as in the Moss, but in very numerous distinct 

 sacs the sporangia. These essential differences of the sporophyte are 

 those which clearly define the Bryophytes from the Pteridophytes. In 

 the latter the mature sporophyte is always a free-growing organism, and 

 a considerable vegetative period usually precedes the formation of the 

 spores. 



Referring back to our observations on the Male Fern in the previous 

 chapter, it will be seen that these remarks apply there also. The most 

 obvious difference between a Lycopod and a Fern is in the size of the 

 leaf; but they correspond in all essentials, 

 and both show a very marked advance 

 of complexity of the sporophyte over the 

 Bryophyte sporogonium. On the other 

 hand, the prothallus of the Fern is a 

 smaller and simpler thing than that of 

 Z. cer?mum, and stands thus in still 

 stronger antithesis to the leafy plant of 

 the Moss. Putting all these points 

 together, it is plain that in the Pterido- 

 phytes the balance in size of the 

 generations is inverted as compared with 

 that in the higher Bryophytes. 



In all the Bryophytes, and also in 

 many Pteridophytes, the spores are all 

 alike, and of small size, as we have seen 

 them to be in Nephrodium and Ly co- 

 podium : this is described as the 

 " homosporous " condition, and it may 

 be accepted as a primitive state. But 

 in certain other Pteridophytes, and in 



all Seed-Plants, there are two different types of spore : the relatively 

 small spore, which is easily transferred when shed, and produces a small 

 male prothallus : and the large spore which, though less easily transferred, 

 develops within it what is so important to the progeny a bulky female 

 prothallus stored with nutriment. This store is derived from the parent 

 plant, and is thus ready to supply the young immediately after fertilisation. 

 The " heterosporous " condition brings a clear advantage, notwithstanding 

 that the separation of the sexes on different prothalli increases the obstacles 

 in the way of fertilisation being carried out. In certain cases the two 

 types of sporangia and spores start their development alike, and only 

 differentiate in the later stages ; for this reason, as well as on grounds 

 of general comparison, the heterosporous state may be accepted as the 

 later and derivative. From the example of Selaginella it will be seen 



FIG. 23. 



Microsporangium of Selaginella apus in 

 median vertical section. Xss- (After Miss 

 Lyon.) 



