222 ACTION OF POLLEN". [BOOK n. 



distinct from the surrounding tissue, much more dense, and 

 coloured ; it is easily separated, and is entirely composed of 

 vesicles of a cylindrical or somewhat fusiform figure, very 

 long, colourless, quite separate at the sides, articulated to 

 each other, end to end, and containing very small regular 

 globules of starch, becoming blue upon the application of 

 iodine. The pollen-tubes which penetrate between the 

 utricles of this tissue are easily distinguished by being much 

 finer, unarticulated, and filled with very fine indistinct gra- 

 nules." (Annales des Sciences.} 



Morren has made some statistical observations upon the 

 sexual organs of Cereus grandiflorus. He found that in each 

 flower of this plant there are about 500 anthers, 24 stigmata, 

 and 30,000 ovules. He estimates each anther to contain 

 500 grains of pollen; the whole number in each flower being 

 250,000 ; so that not more than an eighth of the whole 

 number of pollen grains can be supposed to be effective. 

 The distance from the stigma to the ovules he computes at 

 1150 times the diameter of the pollen grain. 



The exact mode in which the pollen took effect was for a 

 long time an inscrutable mystery. It was generally supposed 

 that, by some subtle process, a material vivifying substance 

 was conducted into the ovules through the style ; but nothing 

 certain was known upon the subject until the observations of 

 Amici and of Adolphe Brongniart had been published. It is 

 now ascertained, that, a short time after the application of the 

 pollen to the stigma, each grain of the former emits one or 

 more tubes of extreme tenuity, not exceeding the 1500th 

 or 2000th of an inch in diameter, which pierce the con- 

 ducting tissue of the stigma, and find their way down to the 

 region of the placenta, including within them the molecular 

 matter found in the pollen grain. These pollen-tubes actually 

 reach the ovules. Brown states he has traced them into the 

 apertures of those of Orchis Morio, and Peristylus (Habenaria) 

 viridis, although this great observer adds that the tubes in 

 those plants probably do not proceed from the pollen. 



Be this as it may, it seems certain that it is necessary for 



