1894- THE CROSS-FERTILISATION OF FOOD-PLANTS. 275 



separated species of a genus the difficulties are greater, and the 

 chances of procuring fertile offspring less ; while hybrids between dis- 

 tinct genera are rare. Out of four hundred experiments in fertilisa- 

 tion of plants of the four different genera — wheat, rye, barley, and 

 oats, with pollen from each other, in only two cases were seeds 

 produced. The two seeds, hybrids between rye and wheat, 

 germinated and produced plants intermediate between the parents, 

 but the pollen did not ripen, and consequently the flowers set no 

 seed. 



In the case of plants which are not cultivated for the sake of their 

 seed, but merely for the flower as by the nurseryman, or for some 

 portion of the vegetative structure like the sugar-cane or the potato, 

 the absence of seed is no disadvantage, while the increased vigour of the 

 vegetative organs may be an advantage. It is obvious, however, that 

 with cereals we must keep well within the limits which will ensure pro- 

 duction of seed ; attempts to produce improved forms must be limited 

 to the crossing of individuals of a single species, including its varieties. 

 Since they have been so long under culture and have given rise to so 

 many cultivated varieties, it is not only hard to determine which 

 variety most nearly represents the primitive form, but also with how 

 many species we are dealing. The following seven types of wheat 

 are easily distinguished: — 



1. Triticum nwnococcum, L., with one grain in each spikelet, and 

 the innermost scale (or pale) dividing into two when ripe. 



2. T. Spelta, L. (spelt), with a loose four-sided ear, and grains 

 which do not fall out in threshing. 



3. T. dicoccum, Schrank, with dense two-sided ears, and grains as 

 in the spelt. 



4. T. vulgare, Vill, with compact ears, and grains faUing when 

 threshed. 



5. T. ttirgidum, L., with dense four-sided ears, and short, thick, 

 blunt grains, which fall out in the threshing. , 



6. T. dumm, Desf., with dense long-awned ears, and long, narrow, 

 very hard, and flinty grains, which fall when threshed. 



7. T. polonicum, L., with long compressed ears, long papery 

 glumes, and long narrow grains. 



Botanists hold different opinions as to the number of species 

 comprised by these seven types. Blomeyer unites them under one, 

 T. sativum, L., while Hackel, the greatest living authority on grasses, 

 thinks T. nionococcum must be separated as a distinct species. The 

 second view is supported by the fact that fertile hybrids have not 

 been obtained between T. monococcwn and any of the varieties of 

 T. sativum. Hackel considers T. polonicum as a species which has, 

 perhaps, arisen under cultivation. Our illustrious countryman 

 Bentham recognised three species, T. inoiiococcum, T. Spelta, and 

 T. sativum, the last being represented by several varieties. Hence, if 

 we wish to obtain improved varieties by means of cross-fertilisation, 



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