LACTUCA SATIVA 619 



The method of pollination is related to the structure and on- 

 togeny of the flower. The stigma develops papillae on its inner 

 surface and these become hairs by the time the stigmatic lobes 

 expand. On the outer surfaces of the stigmas and style, brush 

 hairs are developed; and, since the anthers dehisce before the 

 flower head opens, these hairs gather masses of pollen as they are 

 pushed through the anther cylinder by the elongation of the style. 

 Knuth has stated that following the emergence of the stigmas, 

 "they ultimately roll back into a complete circle, so that automatic 

 self-pollination necessarily results from contact of the stigmatic 

 papillae with the pollen-grains clinging to the sweeping-hairs." 

 Jones found that, in some varieties at least, the backward revolu- 

 tion of the stigmatic lobes was not as complete as reported by 

 Knuth. There were a few pollen grains on the inner stigmatic 

 surface, but the edges of the stigmatic lobes were always covered 

 with pollen as well as the backs and he pointed out that "it is 

 not known definitely whether or not the pollen grains will germi- 

 nate elsewhere on the pistil than on the stigmatic papillae." 



Fertilization and Embryogeny. — Fertilization follows soon 

 after pollination, and at the end of six hours, there may be some 

 two-celled embryos, while in practically all flowers fertilization 

 is completed. In double fertilization the second microgamete and 

 the two polar nuclei unite almost simultaneously to form the 

 endosperm nucleus. 



Jones (13) has investigated the embryogeny in Lactuca and 

 finds that it agrees in general type with that of other Compositae 

 as reported by Carano (5) and Soueges (i6). Following fertiliza- 

 tion, the zygote elongates and develops a definite cell wall; and, 

 shortly thereafter, the first division of the zygote occurs. This is 

 transverse and cuts the zygote into a terminal cell, a, from which 

 the cotyledons and epicotyl develop, and a lower cell, h, which 

 forms the hypocotyl. (Fig. 3x8, A^) In the formation of the 

 four-celled embryo, the cell a forms two cells by a longitudinal 

 division, while cell h divides transversely to form two daughter 

 cells, c and d. (Fig. 3x8, B.) 



In the formation of the eight-celled embryo, the initiation of 

 nuclear activity seems to be apical, and each tier of cells usually 

 divides slightly in advance of the cells below it. In the eight- 

 celled stage, tier a has four cells, tier c two cells, and e and / are 

 derived from the division of cell d. (Fig. 3x8, C.) The i6-celled 



