Nature and development of plants . uitable for their tube cell, stimulated by the secretion of the stigma, rupturesthe outer wall of the spore, which is provided with one or morethin places to favor this growth, and protrudes as a delicate tube, owing to the fact that it is repelled by the oxygen ofthe air, grows down into the tissues of the style which are reallya continuation of the stigma. These tissues of the style are usu-ally looser and provided with abundant foods which are depositedin the cells just ahead of the elongating tube to nourish and directit in its g
Nature and development of plants . uitable for their tube cell, stimulated by the secretion of the stigma, rupturesthe outer wall of the spore, which is provided with one or morethin places to favor this growth, and protrudes as a delicate tube, owing to the fact that it is repelled by the oxygen ofthe air, grows down into the tissues of the style which are reallya continuation of the stigma. These tissues of the style are usu-ally looser and provided with abundant foods which are depositedin the cells just ahead of the elongating tube to nourish and directit in its growth. In this way the tube is directed down the styleto the cavity of the ovary where, owing to the attractive influenceof the organic substances in the sporangium, possibly in the syn-ergids, it usually turns out into the cavity of the ovary, entersthe micropyle and works its way through the sporangium, and,unlike the Pinales, enters the female gametophyte alongside ofone of the synergids (Fig. 273). The antheridial cell usually. Fig. 273. Section of the micropylar end of the megasporangium, show-ing the process of fertilization. The tube, t, has passed through the micro-pyle, entered the female gametophyte and ruptured, discharging the malegametes. One, <$, is shown fusing with the female gamete. $. and theother one, d, is uniting with the two polar nuclei, thus making a triplefusion in the formation of the endosperm nucleus; s, one of the syner-gids ; i, integuments. DEVELOPMENT OF PLANTS 361 divides, forming directly two motionless male gametes during theelongation of the tube, and in other cases the gametes are alreadyformed when the microspores are discharged from their sporan-gia. In none of these cases is there any indication in the divisionof the antheridial cell of the formation of a wall cell as in thegymnosperms, so that the male gametophyte of the Spermatophytapresents a very regular series of reductions from the cycads tothe pines and thence to the angiosperms
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