430 REPRODUCTION. 



1116. When a pollen-grain is deposited upon a fitting stigma, 1 

 at the period when the stigmatic secretion is sufficiently abun- 

 dant, it increases somewhat in size, and soon 2 a tube, 3 sometimes 

 more than one, is thrust forth and passes immediately into the 

 loose tissue of the stigmatic surface. The tube consists of a 

 protrusion of the inline, and its place of emerging is at some 

 one of the perforations of the extine. In some instances the 

 wall separating the larger and the smaller fragments of the 

 original nucleus of the pollen-grain becomes absorbed, and 

 then the two nuclei make their way into the tube as it is 



195 



prolonged. During its descent the pollen-tube is slender, of 

 about the same calibre throughout, and has extremely thin 

 walls. It extends through the conducting tissue of the style, 

 being nourished by the nutrient matter secreted from the cells of 

 that tissue, until it at last reaches the cavity of the ovary. 



1117. According to Capus, 4 the extent of the stigmatic surface 

 bears a definite relation to that of the conductive tissue of the 

 style, one surface being in fact a mere expansion of the other ; 

 and the volume of the conductive tissue of the style is governed 

 by the number of ovules which are to be fertilized. Thus, in a 



1 An interesting account of the artificial fertilization of certain plants of 

 the Poppy family after removal of the stigmas is given by Hooker in " The 

 Gardeners' Chronicle," 1847. It is not known that the experiments have yet 

 been repeated. 



2 According to Gartner, the emission of the pollen-tube begins in some 

 cases in half a minute after the pollen has been applied to the stigma ; but in 

 some others, as in Mirabilis Jalapa and in the Malvaceae, it takes from 24 

 to 36 hours. 



3 Amici, in 1822, appears to have been the first to detect the pollen-tube. 

 His earliest observations were made upon Portulaca oleracea. 



4 Annales des Sc. nat., ser. 6, tome vii. p. 204. 



FIG. 195. Apparatus for cultivating pollen-grains, etc. The object is placed on tlf 

 under side of a glass cover over the circle at a. If necessary, air can be drawn through 

 the tube. A simpler contrivance may be made from a piece of moist pasteboard. 



