266 



PHYSIOLOGICAL BOTANY. 



scribed art. 99- > and the interior being an exceedingly 

 delicate homogeneous pellicle. Whatever may be the 

 ultimate determination of botanists, respecting the form- 

 ation and origin of pollen, yet as its grains in a very 

 early stage of their development are free and unattached 

 to the inner walls of the anther, it should seem that from 

 this period at least their growth must depend upon the 

 absorption of nutriment through their surfaces. 



(262.) Action of Water on Pollen. If ripe pollen 

 be placed in a drop of water and examined under a 

 microscope, in a few seconds it will be seen to dilate, 

 burst, and violently expel a cloud of 

 minute granules (fit/. 1()0.). These 

 granules are still contained within 

 the inner membrane of the pollen 

 grain protruded through the rup- 

 tured outer membrane, but which is 

 difficult to be observed, on account 

 of its extreme tenuity. It thus forms 

 a sort of rude sack, termed a " pol- 

 len tube," and contains a liquid, the " fovilla," in which 

 are dispersed a number of very minute " pollen gra- 

 nules." The outer skin of the grains is ruptured irre- 

 gularly in most Monocotyledons; but in Dicotyledons 

 ^here are one or more determinate points on its sur- 

 face where a regular dehiscence takes place, and it is 

 through these that the inner membrane then protrudes. 

 In consequence of the effect thus produced on pollen by 

 water, it is liable to injury in rainy seasons and the fer- 

 tility of the seed is often impaired. Although the gra- 

 nules are destined to convey that influence to the ovule 

 which is necessary to secure its fertility, yet their violent 

 expulsion from the grains is not the manner in which this 

 effect is produced. This process constitutes one of the 

 most curious phenomena which have been observed of 

 late years among the many wonders which the micro- 

 scope has brought to light. Considering the minuteness 

 of the objects and the delicacy of the inanipulations re- 

 quisite for these investigations, we must feel surprised 



