390 BOTANT. 



a consequence of its maintaining its structural connection 

 with the parent plant, the prothallium (endosperm) is but 

 feebly developed. The prothallium is essentially, as to its 

 function, a nourishing structure, whicli is rendered necessary 

 in the Pteridophytes by the fact that the reproductive bodies 

 separate from the parent plant before they are ready for fer- 

 tilization ; and just as this separation is delayed, or, in other 

 words, just as the parent plant bestows more care upon the 

 bodies which are to give rise to the embryo, so the prothal- 

 lium is less necessary, and, being less necessary, is less de- 

 veloped. Thus we find a much smaller prothallium in the 

 heterosporous orders of Pteridophytes than in the isosporous 

 ones, and in Phanerogams, where parental care extends until 

 after the formation of the embryo, there is generally only 

 the smallest rudiment of a prothallium. 



497. — The leafy plant (which corresponds to the asexual 

 generation of the Pteridophytes) produces two kinds of re- 

 productive cells, viz., pollen grains and embryo sacs, the 

 homologues respectively of microspores and macrospores. 

 The pollen grains are for the most part single cells, which 

 develop from mother-cells in the interior of phyllome struc- 

 tures (modified leaves) ; they soon become free, and are then 

 more or less spherical in shape ; they have two coats, an outer 

 thick one, the extine, and a delicate inner one, the intine, 

 and they contain a granular protoplasm, in which oil drops 

 and starch granules generally occur. The embryo sacs are 

 thin-walled cells which arise axially in the ovules, structures 

 which appear to be homologous to the macrosporangia of 

 Pteridophytes ; they do not become free, but continue to be 

 in organic connection with the cells of the surrounding tis- 

 sues. Each embryo sac develops in its interior a larger or 

 smaller mass of cells, the endosperm, which is the homo- 

 logue of the prothallium, and in which nourishing matters 

 are deposited ; it also develops one or more germ-cells, the 

 homologues of the germ-cells of the archegonia in Pterido- 

 phytes. 



498. — The portions of the plant-body which produce pol- 

 len grains and embryo sacs are in general considerably modi- 

 fied ; thus the axis is generally short, the leaves delicate or 



