liemarJcs on the Impregnation of Plants. 315 



times by three of the grains, and produce as many tubes. The 

 pollen of several plants, however, (particularly in the Cucurbitaceae,) 

 is knovi^n to protrude its inner lining from a great number of points; 

 and Amici has even seen as many as twenty or thirty incipient pol- 

 len-tubes arising from a single grain. Commonly, however, each 

 simple and globular grain of pollen produces but a single tube, 

 which makes its appearance from whatever portion of the surface 

 may chance to be placed in contact with the stigma. This produc- 

 tion can hardly be considered as a mere protrusion of the inner lin- 

 ing of the grain, since the length commonly attained by the tube is 

 greatly disproportionate to the original size of that membrane. It 

 should, perhaps, be regarded as a growth of the inner coat, excited 

 by the fluid which moistens the stigmatic surface. Yet it is hardly 

 probable that this fluid exerts any specific and peculiar agency in the 

 production of the pollen-tube, since it has lately been stated* that 

 a mixture of sulphuric acid and water- causes their production in the 

 same manner as the stigmatic surface itself, only with greater promp- 

 titude. M. Brongniart has also seen them arise from grains of the 

 pollen of Nuphar and some other plants, when floating on water, 

 without having been in contact with the stigma. Usually, however, 

 water is so rapidly imbibed that the grains suddenly burst so as not 

 to admit of their production. The stigma of one plant, moreover, 

 is known to excite the same action in the pollen of diiFerent species, 

 and even of plants belonging to difierent families. Thus, Dr. Brown 

 applied the pollen-mass of a species of Asclepias to the stigma of 

 an Orchideous plant, and found that these tubes were produced as 

 readily as when left in contact with the stigma of the plant from 

 which the pollen-mass was taken. 



The tubes, thus produced in contact with the stigma, penetrate its 

 substance, not. however, by means of any peculiar channel, but by 

 gliding between the cellules and along the intercellular passages 

 which abound in the tissue of the stigma and style. M. Brongniart 

 was able to follow them only for a moderate distance into the tissue 

 of the style, where he thought that they terminated, and, opening 

 at the extremity, discharged the fluid and floating particles of the 

 pollen-grain. He conceives that these larger particles pass along 

 the intercellular spaces into the placenta, and thence into the mouth 



* I have met with this statement in the article Botany, of the Library of Use- 

 ful Knowledge, but I do not know on what authority it rests. 



