314 Remarlcs on the Impregnation of Plants. 



from specimens preserved in an herbarium for more than one hun- 

 dred years. Continuing these investigations, he discovered similar 

 particles, endowed with the same motions when suspended in a fluid, 

 not only in all forms of vegetable tissue, but also in every inorganic 

 substance examined, except those soluble in water or whatever fluid 

 was employed for their suspension.* 



In the year 1823, Prof. Amici, in examining with his powerful 

 microscope some grains of pollen on the stigma of the common 

 purslain, {Portulacca oleracea,) observed that the grains had pro- 

 jected from some part of their surface an extremely slender tube, 

 which was found to consist of the inner lining of the pollen-grain, 

 protruded through a rupture of the external coat. Amici published 

 an account of his discovery in the 19th volume of the Atti della 

 Societd Italiana, whence it was extracted in the second volume of 

 the Annates des Sciences Naturelles. About three years after- 

 wards, these tubes were observed in several plants of different fami- 

 lies by Ad. Brongniart, to whose admirable memoir, published in 

 the 12th volume of the work just cited, we are indebted for the 

 earliest and most complete account of the manner in which they 

 originate and act upon the stigma. 



When grains of pollen fall upon the stigma they are retained either 

 by the hairs with which this organ is often provided, or by its humid 

 and slightly viscous surface ; they slowly absorb this moisture, and, 

 after an interval varying from some hours to a day or more, the outer 

 coat opens by one or more points or slits, through which the highly 

 extensible inner membrane protrudes like a hernial sac, and is slowly 

 prolonged into a delicate tube. The diameter of these tubes does 

 not exceed the 1,500th or 2,000th of an inch, and of course a power- 

 ful microscope is required for their examination. In some plants 

 the grains appear to open at a determinate point, and in numerous 

 instances each one produces two or three pollen-tubes. This hap- 

 pens in the genus (Enothera, and perhaps in all the plants of that 

 tribe, in which the triangular grains open usually by two, and some- 



* For further particulars respecting this curious subject, the reader is referred 

 to the original memoir of Dr. Brown, above cited ; and also to some additional 

 remarks on the same subject, which may be found in a French dress in the 29th 

 vol. of the Annales des Sciences Naturelles. — Respecting the formation of pollen, 

 the reader should consult the memoir of Ad. Brongniart, above cited, p. 21, et 

 seq.; R. Rrown's paper on R,afflesia, in the 12th vol. of the Transactions of the 

 Linnsean Society of London ; and the supplement to Mirbel's memoir on Mar- 

 cliantia polymorpha ia the Nouvelles Mimoires du Miiseuyn. 



