356 



PRACTICAL COURSE IN BOTANY 



In the gymnosperms, pines, yews, cycads, etc., which 

 represent the most ancient and primitive type of existing 



seed-bearing plants, 

 the similarity of these 

 processes to those of 

 certain of the pterido- 

 phytes is very striking, 

 and it was through 

 the study of these that 

 the sequences of the 

 process were traced in 

 the much more obscure 

 form in which they 

 occur among the angi- 

 osperms. From the 

 endosperm in the seeds 

 of gymnosperms arche- 

 gonia were found to be 

 developed (Fig. 510) in 

 much the same way as 



FIG. 510. -Diagrammatic section through the in Selaginella, from the 

 ovule of a gymnosperm belonging to the spruce 



family : i, integument covering the ovule ; e, endo- prothallium, thus 



sperm (corresponding to female gametophyte), Q V inw i rio . fV, p pr ,r|n 



which fills the embryo sac, containing two arche- " 



gonia, a ; o, egg cell ; p, pollen grains ; t, pollen sperm to be a modified 

 tubes entering the neck, c, of the archegonia. 



and greatly reduced 



gametophyte. In some cases, it has even been found to 

 protrude a little way out of the embryo sac and to take on 

 a slightly greenish tinge another reminiscence of its origin. 

 Fertilization, too, takes place in precisely the same manner 

 as in the pteridophytes, except that in all but the ginkgo 

 and the cycads, the fertilizing cells in the pollen grains are 

 non-motile, and find their way to the ovule by growing down 

 into the embryo sac with the pollen tube, instead of swimming 

 to it an adaptation probably brought about in response 

 to changed conditions during the course of evolution from 

 aquatic to terrestrial life. 



