THE FLOWER 129 



Although self-pollination usually occurs just before or at the time 

 of anthesis, cross-pollination is possible, since the glumes remain separated 

 and the flower exposed for 15 to 20 minutes and liable to the introduction 

 of foreign pollen either by the agency of the wind or through transference 

 by small insects, especially thrips, which are frequently found in the 

 flowers. 



More than 1 500 kinds of wheat have been grown annually at Reading 

 during several years, and although self-fertilisation is found to be the rule, 

 five or six natural hybrids are met with nearly every year. These are most 

 frequent among wild T. dicoccoides and some forms of T. vulgar e, but I 

 have observed them occasionally in T. turgidum, T. durum, T. polonicum, 

 and T. Spelta. I have never seen a case of natural crossing in T. 

 monococcum or T. dicoccum. 



Flowers from which the anthers have been removed before maturity 

 frequently produce grain when left unprotected. Salmon in S. Dakota 

 found that over 76 per cent of emasculated flowers left without cover in 

 the field produced grain, but he obtained no grain from a similarly 

 emasculated ear enclosed in a paper bag. 



Obermayer emasculated 29 flowers of an ear of Squarehead wheat 

 and removed all the rest from the ear. Twenty of the emasculated flowers 

 produced grain ; 15 out of 25 flowers of an ear of Hungarian wheat 

 similarly treated " set " grain and 28 out of 34 of an American variety. 



Leighty and Hutcheson also obtained 507 grains from 1240 emascu- 

 lated and unprotected flowers (about 41 per cent) of 70 ears of wheat, 

 and only two grains from 388 similarly treated flowers (about -5 per cent) 

 covered with paper bags. In another series of experiments they observed 

 the production of grain by more than 83 per cent of the emasculated and 

 uncovered flowers, and less than i per cent in the case of emasculated 

 flowers enclosed in paraffined paper bags. 



FERTILISATION AND DEVELOPMENT OF THE GRAIN 



I have not succeeded in inducing the development of the pollen-tube 

 artificially, away from the flower, but in i^ to 2 hours after normal 

 pollination the pollen-tube appears, its tip turns towards the style and 

 pushes its way between the cells into the intercellular space in the centre 

 of the style, along which it grows rapidly downwards towards the ovule. 

 In the cases I have examined I found that the two curved male gametes 

 travel into the pollen-tube before the oval tube nucleus (Fig. 103), and 

 very frequently the latter never leaves the pollen grain. 



The tube, on reaching the cavity of the ovary, finds its way to the 

 micropyle of the ovule, through which it passes, and finally comes into 

 contact with the egg apparatus. About the time of fertilisation the 



K 



