144 Remarks on the 



were unaffected, those of the larger granules were scarcely evident, and 

 often not at all apparent. According to Brongniart's observations, the 

 movements of granules from fresh pollen were suddenly checked when 

 put into alcohol. 



" The smaller particles, or molecules as they are termed by Brown, 

 were first observed by this distinguished naturalist in the pollen of 

 Clarkia pulchella, mingled with the larger granules already described; 

 and they have since been detected in a great number of species. They 

 differ from the larger granules not only by their size, which varies from 

 the 15,000th to the 30,000th of an inch in diameter, but also in their 

 form, which is always spherical, and in their movements, which are 

 oscillatory and extremely rapid. These molecules were also observed 

 in the powder of the so called anthers of mosses and other flowerless 

 plants, by Brown, who found, moreover, that their motions were equally 

 vivid, whether taken from the living plants or from specimens preserved 

 in an herbarium for more than one hundred 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 18-23, Prof. Amici, in examining with his powerful micro- 

 scope some grains of pollen on the stigma of the common purslain, 

 (^Portulacca oleracca,) observed that the grains had projected 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 rup- 

 ture of the external coat. Amici ])nblished an account of his discovery 

 in the 19th volume of the ^illi della Societd Italianu, whence it was 

 extracted in tlie second volume of the Annales des Sciences Naturelles. 

 About three years afterwards, these tubes were observed in several 

 plants of diflferent families, by Ad. Brongniart, to whose admirable me- 

 moir, published in the liith iolume of the work just cited, we are in- 

 delned for the earliest and most comjjlete 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 sliahtly 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 i2,000th of an inch, and of course a powerful 

 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 happens in the genus 

 (Enothera, and perhaps in all the plants of that tribe, in which the tri- 

 angular grains oi)en usually by two, and sometimes by three of the 

 grains, and ])roduce as many' tubes. The pollen of several plants, 

 however, (particularly in the Cucurbitacese,) is knownto protrude its 

 inner linina- from a great number of points; and Amici has even seen 

 as many as^twenty ov thirty incipient jjollen-tubes arising from a sin<;le 

 grain. ConmionlV, however, each simple and globular grain of pollen 



*" For further particulars respecting lliis curious subject, the rca<ler is referred to the 

 original memoir of Dr. Brown, above cileil ; and also to some additional remarks on the 

 same subject, wiiich may be found in a French dress in the 29ih vol. of ihe Annales des 

 Sciences Naturelles. Respecting the formation of pollen, ilie reader shoidd consult the 

 memoir of Ad. Brongniart, al)ove cited, p. 21. et seq. ; K. Brown's paper on Rafflesia, 

 inlhel2ih vol. oftlie'l'ransaclionsof the Linmran Society of London ; and the supple- 

 ment to Mirbel's memoir on Marchanlia polymorpha iu llie Nouvcllcs Mimoircs du Mu- 

 sium" 



