REPORT ON THE PHILOSOPHY OF BOTANY. 47 



celebrated Prussian traveller and botanist Ehrenberg had dis- 

 covered that the grains of pollen of Asclepiadece acquire a sort 

 of tails which are all directed to a suture of their sac on the side 

 next the stigma, and which at the period of fertilization are 

 lengthened and emitted ; but he did not discover that these 

 tails are only formed subsequently to the commencement of a 

 new vital action connected with fertilization, and he thought 

 that they were of a different nature from the pollen-tubes of 

 other plants ; he particularly observed in Asclepias syriaca 

 that the tails become exceedingly long and hang down. 



In 1831 the subject was resumed by Dr. Brown* in this 

 country, and by M. Adolphe Brongniartf in France, at times so 

 nearly identical, that it really seems to me impossible to say 

 with which the discovery about to be mentioned originated : it 

 will therefore be only justice if the Essays referred to are spoken 

 of collectively instead of separately. These two distinguished 

 botanists ascertained that the production of tails by the grains 

 of pollen was a phaenomenon connected with the action of ferti- 

 lization ; they confirmed the existence of the suture described 

 by Ehrenberg ; they found that the true stigma of Asclepia- 

 dece is at the lower part of the discoid head of the style, and 

 so placed as to be within reach of the suture through which the 

 pollen-tubes or tails are emitted ; they remarked that the latter 

 insinuate themselves below the head of the style, and follow its 

 surface vmtil they reached the stigma, into the tissue of which 

 they buried themselves so perceptibly that they were enabled to 

 trace them, occasionally, almost into the cavity of the ovarium ; 

 and thus they established the highly important fact, that this 

 family, which was thought to be one of those in which it was 

 impossible to suppose that fertilization takes place by actual 

 contact between the pollen and the stigma, offers the most 

 beautiful of all examples of the exactness of the theory, that it 

 is at least owing to the projection of pollen-tubes into the sub- 

 stance of the stigma. In the more essential parts these two 

 observers are agreed : they, however, differ in some of the de- 

 tails ; as, for instance, in the texture of the part of the style 

 which I have here called stigma, and into which the pollen- 

 tubes are introduced. M. Brongniart both describes and figures 

 it as much more lax than the contiguous tissue, while on the 

 other hand Dr. Brown declares that he has in no case been able 

 to observe " the slightest appearance of secretion, or any dif- 



* Observations on the Organs and Mode of Fecundation of Orchideae and 

 Asclepiadece. London, October 1831. 



f Annft/fis des Sciences for October and November 1831 ; from observations 

 made in July, August and September of that yoar. 



