146 MORPHOLOGY OP ANGIOSPERMS 



tion." Nevertheless, in his text-book, published a few years 

 later, he says that " fertilization " is accomplished in plants, 

 as in animals, by the union of male and female elements. 



It is only since 1875 that detailed information has gradu- 

 ally accumulated; and not until 1884 (Strasburger 8 ) were the 

 cells concerned in fertilization clearly pointed out (Fig. 66). 



The tube-cell of the pollen-grain in various ways pushes 

 through the exine a papillate protrusion of the intine that 

 develops into the pollen-tube with greater or less rapidity. 

 Crowding among the loose papillate cells of the stigma, the 

 elongating tubes enter the conducting tissue of the style. Ordi- 

 narily the style is solid, and the tubes grow along the conducting 

 strand, which they disorganize more or less, obtaining from it 

 their nutritive supply. In case there is a stylar canal the tubes 

 either pass down it, as in Pontederia (Smith 28 ) and Erythro- 

 nium (Schaffner 51 ), nourished by the lining glandular cells, 

 or they may penetrate the stylar tissue about the tube, as in 

 Campanula (Barnes 9 ) and Juglans (Nawaschin 20 ). In many 

 cases the tube enters the ovary cavity close to the micropyle; 

 in others it must traverse more or less of the cavity, being 

 " guided " to the micropyle by various mechanical and nutri- 

 tive contrivances. 



Although ordinarily pollen-tubes are developed only in con- 

 tact with the stigma, in cleistogamous flowers tubes have been 

 observed issuing from pollen-grains still in the anther, the tips 

 being directed toward the stigma. In Asclepias also multi- 

 tudes of tubes sometimes start from the unremoved pollinia. 



The time elapsing between pollination and fertilization, as 

 inferred from the presence of pollen-tubes in the embryo-sac, 

 is extremely variable, and seems to hold no relation to the dis- 

 tance traversed, as shown by Hofmeister, 3 in comparing Crocus, 

 in which a style 6 to 10 cm. long was traversed in one to three 

 days, with Arum, in which a style only 2 to 3 mm. long was 

 traversed in five days. The range in time is probably repre- 

 sented by the following illustrations: In Limnocharis emargi- 

 nata Hall 57 found a two-celled embryo in material killed 

 eighteen hours after pollination, and thinks that in this case 

 fertilization probably occurs the first night after pollination. 

 Probably the most accurate estimate of the time is that by 

 Mottier 26 for Lilium, in which the time between artificial pol- 



