750 



LECTURE XLII. 



true prothallus is separated off from the large spore-cavity by the diaphragm d d: in 

 this cavity there arises a large-celled tissue, such as we shall meet with later in the 

 embryo-sac of flowering plants — the so-called Endosperm. Figure /, then, shows at 

 / a young embryo, developed from a fertilised oosphere, and at e one already further 

 developed, the shoot portion of which (only roughly indicated) has bored into the 

 endosperm tissue, but which subsequently grows out from it again. 



In order to proceed hence to the formation of the seed in the Coniferae, we need 

 only assume that the macrospore does not open at all, the prothallium and endosperm 



Fig. 435.— Germination of Setaginella (after Pfeffer). /— /// .? Marlensu, 1 — D S <,!:,Us tiis 

 I lonffitudinal section of a macrospore filled with prothallus and endosperm (<r aiapnragm) m wmcn 

 two embryos e e' are developing. // a young archegonium, not yet opened. /Jl an archegonium 

 containing a fertilised oospore, which has undergone one division. A a microspore, showing the 

 divisions of the contents. B C different views of these divisions. D mother-cells of antherozoids, in 

 the mature antheridium. 



arising in its interior, and one or two embryos becoming developed. Of course, in 

 the case of the Conifer® and all Gymnospermous plants, what actually occurs is not 

 only what has just been mentioned, but moreover the macrospore itself also in which 

 these processes occur remains lying in the very massively developed sporangium, and 

 no opening of any kind exists through which antherozoids for instance could penetrate 

 to the oosphere. Here, in fact, we reach a turning-point in the history of fertilisation ; 

 in that not only in the case of the Gymnospermous plants (Coniferse, Cycadese), but 

 even in a still higher degree in the true flowering plants, the oospheres remain com- 

 pletely enclosed in masses of tissue, and thus the possibility of their fertilisation by 



