Dr. G. Dickie on the Physiology of Fecundation in Plants. 7 



adhesion to the other coats, I found the tubes to enter the mouth 

 of the secundine, but after repeated observations was unable to 

 detect any such connexion as that described by Meyen to occur 

 between the end of the pollen-tube and the embryo-sac in some 

 species of Mesembryanthemum, &c. ; instead of which, the tube 

 and the apex of the nucleus (as at present I suppose it to be) 

 seemed perfectly continuous, the one being apparently a gradual 

 prolongation of the other. This therefore induced me to ex- 

 amine the tubes in their whole course, and there were seen what 

 I supposed to be the same tubes, terminating upwards in blind 

 extremities. In one capsule the pollen grains which had fallen 

 on the stigma were very few, while the ovules (in the same cap- 

 sule) were most of them supplied with the organs described ; I 

 concluded, therefore, that they were not at all connected with the 

 pollen. The position of the exostome in reference to the apex of 

 the placenta (it is directed to the base of that organ, and conse- 

 quently away from the descending pollen-tubes) appeared also 

 to present an insurmountable objection, unless we suppose the 

 pollen-tubes to be possessed of a remarkable instinct, which 

 I presume physiologists would hesitate to ascribe to them. 

 They must, in order to reach the nucleus after descending, find 

 their way to the exostome and then ascend, passing along the 

 canal leading to the endostome. The smallness of the aperture 

 in the outer membrane and its irregular margin would render 

 it difficult for a pollen-tube to hit upon it exactly, keeping out 

 of view altogether its direction. On the outside of the tubes 

 described, were numerous spherical molecules, some of which 

 were in active motion, and occasionally a few similar particles 

 were seen in their interior. I afterwards found that these mo- 

 lecules were abundant in the tissue of the placenta. The opi- 

 nion first entertained about these organs {ovule-tubes), viz. that 

 they had their origin from the pollen, was for these reasons 

 abandoned. Supposing them to be prolongations of some part 

 of the ovule itself, an objection having reference to the position 

 of the exostome, and the passage of the tubes upward towards 

 the apex of the placenta, might with great justice be urged ; I 

 found, however, that they in common have a great tendency to 

 become tortuous and bent upon themselves : such a tube there- 

 fore, on issuing from the exostome, and on being bent from its 

 original direction, would come in contact with the placenta, and 

 might have its further course regulated by contact with that or- 

 gan. Without however laying much stress upon this, or ven- 

 turing to speculate more in the matter, it may be remarked that 

 the mode of growth of the tube would assist materially in regu- 

 lating its course, especially if its increase in length took place at 



