W. H0PMEI8TER ON THE DEVELOPMENT OF ZOSTERA. 251 



The end of the pollen-cell penetrates into the canal of the 

 style opening at the summit of the two arms of the stigma. I 

 was unable to lay free, uninjured from the stigma to the micro- 

 pyle, the pollen-tube into which the otherwise already tubular 

 pollen-cell is doubtless converted by continued growth of one of 

 its extremities in the longitudinal direction. Within seven 

 hours after the dehiscence of the anther, the pollen-tube is found 

 in the cavity of the ovary * ; adhering closely to the outer side 

 of the pendent ovule, it grows down to its micropyle, into which 

 it penetrates, suddenly making a sharp curve. The internal 

 cavity of the ovary, now greatly enlarged by the expansion of 

 its walls, is filled with transparent but tolerably firm jelly, in 

 which may sometimes be distinguished swollen-up cells, not un- 

 like those of the epithelium of the inside of the human mouth. 

 They are perhaps detached cells of the conducting tissue. A 

 little air-bubble is commonly found in the lower extremity of 

 the cavity of the ovary, at the end opposite to the stigma. 



The pollen-tube is of the same diameter as the pollen-cell 

 (figs. 24-26). The portion outside the micropyle dies away 

 rapidly; within this it remains perceptible for a considerable 

 time. It does not usually penetrate farther than the summit 

 of the embryo-sac, very rarely insinuating itself for a short 

 distance down between this and the inner wall of the second 

 integument. 



After the arrival of the end of the pollen-tube at the outer 

 wall of the embryo- sac, formed by the enveloping membrane of 

 the nucleus, one of the germinal vesicles increases in size and its 

 nucleus vanishes (fig. 24). The other germinal vesicles shrivel 

 up (fig. 25) and die away ; often even before the pollen- tube 

 has emerged from the micropyle-canal, A newly-formed, glo- 

 bular or ellipsoidal nucleus soon makes its appearance in the 

 lower end of the fertilized germinal vesicle, inside the mass of 

 protoplasm accumulating there ; immediately after this comes, 

 above the nucleus, a septum, convex on the upper side, dividing 



* Anthers of a plant of Zostera marina taken from the sea-water on the 

 4th of June 1851, opened before my eyes on the 7th of June at half-past 6 in 

 the morning, after a land journey of more than forty hours. By one o'clock on 

 the same day the pollen-tubes had penetrated into the micropyles of ovules of 

 the same inflorescence. The plants had been kept ever since their removal from 

 their habitat moderately damp (not wet), and excluded from the air. 



