MULTIPLICATION IN PLANTS 473 



sometimes called the oosphere or the egg cell ; the small one 

 is called the spermatozoid or the sperm cell. We sometimes 

 call the large gamete, or egg cell, the female, and the smaller, 

 or sperm, cell the male. 



]Most of the familiar plants and animals reproduce by means 

 of male and female gametes, forming zygotes. This kind of 

 reproduction is called sexual reproduction, in distinction from 

 reproduction by spores, which is called asexual (that is, without 

 sex), and in distinction from vegetative propagation. 



There are many species of animals and plants that reproduce 

 both sexually and asexually. 



348. Flowers. If we recall the steps in the formation of seeds in 

 flowers, we can understand the meaning of some of the structures and 

 processes from a new point of view. The pollen grains and the embryo 

 sacs in the ovules are considered spores. The changes that take place in 

 the protoplasm of these two structures lead to the formation of gametes. 

 One of the nuclei at the tip of the pollen tube (sect. 35) corresponds to 

 the male gamete, or sperm cell. One of the nuclei resulting from the 

 cell division inside the embryo sac corresponds to the female gamete, or 

 egg cell. The fusion of these two nuclei is the essential step in fertiliza- 

 tion ; it is a union of gametes. The resulting nucleus is a zygote and 

 proceeds at once to divide, starting a new plant. After cell division has 

 proceeded for some time, the embryo, or voung plant, shows differentia- 

 tion of parts into shoot and root (hypocotyl, cotyledons, and epicotyl). 

 The seed which results from the ovule is thus the embryo, belonging to 

 one generation, plus the coverings, belonging to the parents of the 

 spores. The parent of the pollen is of course not present in the tissues 

 of the seed, but the parent of the embryo sac furnishes not only the 

 protective coverings but also the stored food material, as well as all the 

 food which nourished the embryo from the moment the zygote was 

 formed to the time when the seed is ripe. 



349. Fruit. The fruit, which is derived chiefly from the ovary, 

 is significant in the life of the plant only for its relation to the 

 succeeding generations. It is commonly a protection of the 

 young plant while the seeds are ripening : and it is often in dif- 

 ferent ways an aid in the distribution of seeds after they are 

 ripe (see Figs. 34 and 195). 



