ONTOGENESIS 181 



the tube-cell, and a smaller one which divides again 

 into two male gametes. The tube-cell, in its growth 

 down the style, clears the way for the two gametes, 

 and when it has entered the embryo-sac, it swells 

 and ruptures, discharging the two gametes, one of 

 which conjugates with the egg-cell, the other with 

 the endosperm cell. 



The zygote, formed by the fusion of these two 

 gametes, develops into the embryo of the plant. 

 Meanwhile, the endosperm cell also divides and 

 forms a growing tissue that packs about the embryo. 

 The food (oil, starch, etc.) contained in the endo- 

 sperm nourishes the embryo plant until it is able to 

 get its own food. Finally, the outer layers of the 

 ovule secrete various kinds of hard protective cover- 

 ings which form the seed-coats, and the young plant, 

 thus wrapped up, is cast off as a seed. In this way, 

 unfavorable seasons are tided over. In some cases 

 the carpels themselves, or tissues adjacent, become 

 soft and succulent, forming fruits that inclose the 

 seed. When suitable conditions of temperature and 

 moisture again intervene, the sporophyte within the 

 seed resumes its growth at the expense of the stored- 

 up food, and, bursting its seed-coats, grows out, 

 takes root, and resumes the cycle. The embryo 

 has a root, a stem, and one or two leaves called 

 cotyledons. In those seed-plants with one cotyle- 

 don the monocotyledons (palms, lilies, orchids, 

 etc.) the leaves are parallel-veined, there is a 

 " pithy " stalk (like that of corn), and, usually, 

 flower-parts arranged in " threes." In those with 



