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TEXTBOOK OF BOTANY 



spore leaves (stamens and pistil), as well as the sepals 

 and petals, are borne at nearly the same level. 



b. The seeds of the pine are borne on the exposed surfaces 

 of the macrospore leaves ; those of the bean are en- 

 closed within the ovary, which has been formed by a 

 folding of the macrospore leaf. 



c. The sexual generation, already much reduced in the 

 pine, is reduced still further in the bean. The female 

 plant now consists of only seven cells, the male plant 

 of only three. 



d. The endosperm of the pine is formed before fertiliza- 

 tion; it is the vegetative tissue of the female plant. 

 That of the bean develops after fertilization from a cell 

 whose nucleus has been formed by the union of two 

 nuclei of the female plant with one nucleus of the male 

 plant ; it disappears before the seed is ripe ; but in 

 the seeds of the com (see Chapter XIII) and in those of 

 many other angiosperms, the endosperm is still present 

 in the ripe seed, just as it is in the seed of the pine. 

 The life cycle of the bean may be represented by a dia- 

 gram substantially like that used for the pine (Fig. 77). 



176. Species and Varieties. — The name " bean " is applied to 

 many very different plants. But nearly all the plants that are culti- 

 vated for food in the United States and Canada under this name are 

 varieties of two species of the genus Phaseolus. Most of the common 

 field beans and garden beans belong to the species Phaseolus vulgaris; 

 they are sometimes classed together as " kidney beans.'' A list of 

 145 distinct varieties of kidney beans grown in the United States was 

 prepared in 1907, and it is estimated that in all, throughout the world, 

 there are at least 500 varieties. Among the kidney beans are both 

 low and climbing forms, and they differ greatly from one another in 

 the color, shape, and size of their pods and seeds, as well as in various 

 other ways. P. lunatus includes the Lima beans, of which there are 

 perhaps fifty varieties, some low and some climbing. Lima beans 

 are not so much grown in other parts of the world as they are in the 

 United States. Several others of the 150 or more species of Phaseolus 

 are cultivated in various parts of the world. P. nndtiflonis includes 



