184 BOTANY: PRINCIPLES AND PROBLEMS 



another, and a colony of plants takes the place of a single indi- 

 vidual. In a somewhat similar fashion the beech tree multiplies 

 itself, buds arising on the roots of the parent tree imtil it is 

 surrounded by a grove of beeches. This method of reproduc- 

 tion, however, is not very common among the seed plants. 



In many species structures have arisen which are particularly 

 adapted for aiding vegetative dispersal of the plant body and 

 which thus partake somewhat of the nature of reproductive 

 structures. To this group belong the runner or stolon of plants 

 like the strawberry, the rapidly spreading rootstocks of the quack 

 grass, and the long, arching stems of the blackberry, in which the 

 tips touch the ground and there take root. Other vegetative 

 parts are sometimes modified still further as reproductive 

 organs. Perhaps the best known example of such is the tuber 

 (Fig. 46) of the potato, which is merely a short and very much 

 thickened underground stem, from the buds of which new potato 

 plants arise the next season. The bulb (Fig. 47), as in the onion 

 and hyacinth, and the corm (Fig. 47), as in the crocus, are also 

 short, stout stems with their lower leaves modified as scales. 

 They carry the plant over from one season to the next and their 

 buds ultimately give rise to a group of new plants. 



Sexual Reproduction. — Far commoner and more important 

 than the asexual or vegetative method of reproduction, however, 

 is the sexual. The essential feature here is the union of two special- 

 ized sexual cells, or gametes, to form a single cell, the fertilized egg 

 or zygote, from which a new individual develops. To insure the 

 successful consummation of this process is the function of a great 

 variety of reproductive structures throughout the plant kingdom. 

 These in the higher plants we call the flower, fruit, and seed. We 

 are still uncertain as to what notable advantage is gained through 

 this process of sexual union which should make it so common 

 among plants. There is evidence in some cases that an increase 

 in vigor characterizes the offspring of a sexual union, but just how 

 valuable this advantage is we do not know, for there exist many 

 very successful species which now depend wholly or in great 

 part upon asexual reproduction. 



In the present discussion we shall consider only the higher 

 seed plants and shall reserve a study of reproduction in the 

 lower members of the plant kingdom for later chapters. 



The Flower. — Seed plants are characterized by the possession 

 of a rather complex reproductive apparatus known as the flower. 



