Impregnation of Plants. 145 



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 production can hardly be considered as a mere protrusion 

 of the inner lining of the grain, since the length commonly attained by 

 the tube is greatly disproportionate to the original size of that mem- 

 brane. 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 aii?e from grains of the pol- 

 len 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 different species, and even of 

 plants belonging to diflTerent families. Thus, Dr. Brown api)lied 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 betwen 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 intercelhilar spaces 

 into the placenta, and thence into the mouth of the ovules. Prof. Amicif 

 has, however, recently announced that he had inseveral instances traced 

 the pollen-tubes themselves quite into the cavity of the ovary; from 

 which he infers that the immediate contact of this body with the mouth of 

 the ovule takes place whenever impregnation is eflfected. 



" In the autumn of the year 1831, Dr. Brown read before the Linnsean 

 Society of London his highly interesting memoir on the Organs and 

 mode of Fecundation in "the Orchidece and Jlsdepiadece ; which has 

 since been published in the last volume of the transactions of that soci- 

 ety. It is unnecessary for our present purpose to indicate the several 

 curious and important results of the investigations of that sagacious bo- 

 tanist, relative to the structure and impregnation of these two families. 

 He followed the course of the pollen-tubes, in several plants of both 

 orders, from the stigma to the placenta, and in a single instance traced, 

 in an Orchideous pFant, some tubes or vessels of equivocal nature, quite 

 into the aperture of the ovule. Dr. Brown reroiirks that these tubes 

 had been noticed in the style and ovary of these two families many 

 years previous to his observations, viz. in Orchideous ])lants by Du 

 Petit-Thouars, as early as 1816 or 1818; and by the late Mr. Elliott in 

 Podostigma, (a ijenus of Asclepiadete,) as stated in the first v(^lume of 

 the Sketch of the Botany of South Carolina and Georgia, jiublislied in 

 1B17. Mr. "Elliott adds tliat Dr. Macbride (since decease<l) had obser- 

 ved the same fibres or cords in the style of some species of Asclepias. We 



* " I have met with this statement in the article Botany, of tlie Library of Useful 

 Knowledge, but I do not know on what autliorily it rests." 



t" Extract from a letter from Piof. Aniici to M. IMirbcl, dated 3d July, 1S30, and 

 published in the 21st vol. of tlie Annales des Sciences Natui'eUes." 



VOL. III. NO. IV. 19 



