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THE POPULAR SCIENCE MONTHLY. 



small male spore of the higher cryptogams takes the name of pollen- 

 grains, and the larger female spore is known as the embryo sac. The 

 latter does not sever its connection with the mother-plant until after 

 an embryo plant has formed. On this account the prothallium — which 

 we have seen as an independent structure in ferns and diminishing 

 gradually as we ascended in the scale of flowerless plants — is here but 

 feebly developed. The flower-bearing plant, whether herb, shrub, or 

 tree, is the asexual generation producing two kinds of structures, 

 which, by their development and union of parts, produce a plant like 

 the one from which the sexual generation sprang. The pollen-grain is 

 usually a small spherical or oval body that, when mature, separates 

 from the case (anther) in which it was formed. Figs. 16 shows the 

 form of some simple pollen-grains. Grain A shows the rudimen- 

 tary prothallium as a small cell, 

 y ; B is a pollen -grain forming 

 the tube. Much the same is seen 

 at C and D, excepting that the 

 prothallium is made up of three 

 small cells. In structure and func- 

 tion these pollen-grains are almost 

 identical with the male spores of 

 higher cryptogams. The embryo 

 sac is more or less surrounded by 

 the substance of the parent-plant, 

 and develops within itself a pro- 

 thallus of small size which is 

 known as the endosperm, and is a 

 store-house of nourishing matter 

 for the young embryo. One or 

 more cells form the homologue of 

 the archegonia in higher cryptogams with its female germ. The male 

 cell or pollen-grain no longer develops a number of mobile, fertilizing 

 antherozoids or spermatozoids ; but, instead, the whole pollen-grain 

 passes to a receptive surface (stigma) situated somewhere near the 

 female organ, from which it sends out a tube that penetrates the tissue 

 provided for its passage (style) until it reaches its destination and 

 mingles its contents with those of the female cell. Circumstances ob- 

 tain in the flowering plants which render mobile bodies like spermato- 

 zoids worthless as a means of fertilization. In many cases the male 

 element needs to pass from one tree to another, and even from one 

 country to another. The first obsei'ved result of fertilization is the 

 formation of the suspensors, mentioned under Selaginella. At the 

 lower end of the suspensor the young plantlet is formed with its one 

 or more small seed-leaves and a short root and stem. In this growth 

 the food-material in the endosperm is frequently entirely exhausted. 

 The ovule, as the female cell with its immediate surrounding tissue is 



Fig. 16. 



