DESCENT OF THE POLLEN-TUBE. 431 



pistil with a large number of ovules the stigmatic surface is 

 large, as is also the amount of conductive tissue of the stj-ie 

 through which the pollen-tubes are to descend. 



1118. The conductive tissue through which the pollen-tube 

 descends, and by which it is nourished, is formed at the stigma 

 by a modification of epidermal ceUs, and below this arises from 

 modifications in the parenchyma ; in the style it may constitute 

 a solid mass of delicate cells, sometimes with walls which have 

 undergone the mucilaginous modification, or it may simply line 

 the hollow tube which is frequently found, as in the pistil of 

 the violet. 



1119. The time required for the descent of the pollen-tube de- 

 pends upon the length and character of the path the tube is 

 to traverse, and is verj- different in 

 different cases. Hofmeister states 

 that in Croons vernus, with a style 

 which is from one to two inches in 

 length or sometimes more, the tube 

 reaches the ovary in from one to 

 three days. Schleiden ^ gives the 

 following times required for descent 

 of the tube : Cereus grandiflorus, 

 having a style nine inches long, a 

 few hours ; Colchicum autnmnale, 

 with a stj'le thirteen inches long, 

 twelve hours. In some other cases 

 (certain orchids) it is weeks before 

 the end of the tube lias descended 

 for even a very short distance. 



1120. A single pollen-grain of 

 some flowers can emit more than 

 one pollen-tube : thus Amici has 

 seen twent}' to thirty' tubes proceed 

 from one grain. Pollen-tubes sometimes branch in their course 

 downward. 



1121. The length of tii^ae during which pollen-grains can 

 preserve their vitality has been determined for a few cases : ^ 



1 Schleiden: Principles of Scientific Botany, 1849, p. 407. 



2 Gartner, quoted by Mohl: Vegetable Cell, p. 134. 



Fig 196. Diagram of a longitudinal section of an nvary liaving only one ovule with 

 basal placentation, designed to exhibit the course of the pnllen-tube from the stigma to 

 the summit of the embryonal sac above the oosphere. Tlie ovule is auatropous, and 

 is inserted, as is usually the case in Composite. (Luerssen.) 



