BOTANICAL CHARACTERS 



T07 



*»d 



with the nucleus of the egg-cell to form the embryo (Fig. 54) ; 

 fertilization is then accomplished. The other male nucleus 

 fuses with the two fused female polar-nuclei ; from these the 

 endosperm is developed. Cell-growth and cell-division then 

 commence, and are continued until the mature seed is developed. 

 The chromosomes of maize are small and difficult to study, 

 and scarcely anything is known of their behaviour during the 

 maturation division {East, 6). 



79. Dichogamy.— According to Kerner ( 1 ), most moncecious 

 plants, including maize, are proto- 

 gynoits, i.e. the female flowers are 

 receptive before the pollen of the 

 same plant is shed, thus necessitat- 

 ing cross-pollination. If protogyny 

 were complete, the very first plant 

 in a field, which developed female 

 flowers, would, in consequence, 

 fail to propagate its kind, unless 

 accidentally pollinated from a still 

 earlier plant in a neighbouring 

 field ; but there would always be 

 oiie plant in a district that was 

 earliest of all, and which would 

 therefore fail to develop grain. 

 This habit would act to some 

 extent as a check to any natural 

 tendency to increased earliness in 

 maturity. 



If all the plants in a field 

 flowered on exactly the same 

 day, and all were completely 

 protogynous, there would be no pollination except from other 

 and earlier-planted fields, and those of the first planting would 

 always fail to develop grain. But this is not the case. As 

 already pointed out (If 72), there is a great difference in time 

 of flowering with individuals in the same field, due to many 

 causes, e.g. individual characteristics, difference in depth of 

 planting, variation in soil fertility, soil texture and soil moist- 

 ure, etc. The flowering period in any one field or plot may 

 thus extend over ten days or three weeks. 



Fig. 54. — The embryo-sac in 

 maize at the time of fertilization. 

 PI., pollen tube which has just dis- 

 charged the two male nuclei, o* ; 

 $ , egg-cell which, after union with 

 one of the male nuclei, forms the 

 embryo; cud., nucleus of the en- 

 dosperm, with which the second 

 male nucleus may unite. (After 

 drawing by F. E. Lloyd in Dug- 

 gar's Southern Field Crops, The 

 Macmillan Co.) 



CHAP. 

 IV. 



