w 
314 Remarks 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 Aéti della 
Societa Italiana, whence it was extracted in the second volume of 
the Annales 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 toa 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 i is required for their examination. In some plants 
the grains appear to open at a determinate point, and in numerous 
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 en oe this curious subject, the —_ is referred 
to the original memoir of Dr. Brown, above cited; and also to some additional 
remarks on the same 2 sabpeed ‘which may be found in a French port 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 Rafflesia, in the 12th vol. of the Transactions of the 
Linnean Society of London; and the supplement to Mirbel’s memoir on Mar- 
chantia polymorpha in the Nouvelles Mémoires du Muséum 
