Metaphyta Selaginella. 1 39 



parable to the cells found in the cavity of the ovarium of the 

 fern thallus. There are in each cavity one large basal cell? 

 or ovum, and one or more cells filling up the neck, i.e. canal 

 cells. We have no option, then, but to consider this so- 

 called macrospore as a thallus which is developing its ovaria 

 within the spore-wall. The entire second or sexual gene- 

 ration has remained hidden in and protected by the spore- 

 wall. What shall we say then of the microspore viewed in 

 the light of this discovery ? Simply that here, too, the thallus 

 has never left the spore-wall, but has first of all divided 

 into a vegetative cell, probably of no service save to guide 

 us towards an explanation of this anomaly, and a number 

 of reproductive cells which at once proceed to form sperms 

 without forming spermaria at all. 



It appears, therefore, that we have in the spores of Seia- 

 ginella a great advance on the condition of affairs in the 

 fern. In the first place we find two kinds of thalli instead 

 of one, and these ihalli are unisexual instead of herm- 

 aphrodite. How is this to be explained ? Probably in terms 

 of a law which we shall find of supreme importance in higher 

 plants, namely, the law of cross-fertilisation. It is an ad- 

 vantage for a plant to be fertilised by male elements derived 

 from another plant than itself. In the fern, because the 

 thallus was a comparatively large organism and because it 

 obtained its nourishment from the environment, probably 

 the protoplasm of the thallus was sufficiently differentiated in 

 the region of the ovaria from the protoplasm in the region 

 of the spermaria to allow of the invigorating effect of inter- 

 crossing being thus obtained. Possibly future experiments 

 may show that true cross-fertilisation is as helpful t6 the 

 Algae and the Non-vascularia as it is in the case of the 

 flowering plants ; indeed there are numerous observations on 

 record pointing to this conclusion. In Selaginella, since the 

 thalli are not independent living plants, we have male and 

 female elements produced in different thalli, which are pro- 

 duced from different spores, instead of being developed, as 



