2 94 TEXTBOOK OF BOTANY 



exceptions ; for certain seeds, if dry, may be exposed for some 

 days to a temperature of about 480 degrees below zero 

 Fahrenheit without destroying their power of germination. 



" Seed-testing " consists in placing a considerable number 

 of seeds, taken as a sample from a large lot, under conditions 

 favorable for germination — for example, between sheets of 

 blotting paper, in a germinating box, or in soil — and noting 

 the percentage of the whole number that germinate success- 

 fully. It is important for the farmer or the gardener to be 

 sure that the great majority of the seeds that he sows are 

 likely to germinate, and for this reason various methods of 

 practical seed-testing have been devised. 



297. Development of Seeds or Fruits without Fertiliza- 

 tion ; Seedless Fruits. — In nearly all seed plants, no embryo, 

 and therefore no seed or fruit, can be formed unless the egg is 

 fertilized by a male nucleus. Some exceptions to this rule, 

 however, are known to occur. The eggs of a few plants, 

 among them the common dandelion, can develop into 

 embryos without fertilization.- Such plants, therefore, can 

 form seeds and fruits without a previous fertilization and 

 even without pollination. Certain other plants, including at 

 least some varieties of cucumbers, produce fruits \\dthout 

 pollination or fertilization ; but a cucumber fruit so produced 

 contains no developed seeds. A considerable number of 

 plants, including apples, pears, and lemons, bear occasional 

 seedless fruits ; but in most such instances, differently from 

 the case of the cucumber, it is probable that pollination and 

 fertilization actually occur, but that the seeds for some reason 

 fail to develop. By selection, species of seedless oranges and 

 other fruits have been developed which are much better for 

 eating than fruits with seeds. Seedless varieties, of course, 

 can be propagated only by vegetative means, as by cuttings or 

 grafts. The common banana has been seedless for centuries. 



