244 W. HOPMEISTER ON THE DEVELOPMENT OF ZOSTERA. 



is not, however, altogether an isolated phaenomenon ; the earlier 

 stages of formation of the pollen-masses of the Asclepiadaceae 

 resemble it most perfectly. In the anther of the Asclepiadaceae, 

 the laws of growth of which correspond on the whole to those 

 of the rudimentary spadix of Zostera, and which, like this, is 

 at an early age shaped like a little sloped-off spatula with a 

 strongly rounded back, two groups of longitudinal rows of cells 

 become the primary parent-cells of the pollen*. Extending 

 considerably inwards in a direction perpendicular to the surfaces 

 of the anthers, they acquire a different character from the sur- 

 rounding tissue and assume the form of recumbent prisms. By 

 a series of longitudinal and transverse divisions parallel to the 

 long axes of these prisms, the rudiment of the pollen-mass be- 

 comes a group of narrow cells of a length six to ten times greater 

 than the cross diameter. At this stage of development it cor- 

 responds perfectly to the contents of a chamber of the young 

 anther of Zostera. But in the Asclepiadaceas there now succeeds 

 a many times repeated division of the elongated parent-cells, by 

 means of walls perpendicular to their long axes, whereby the 

 pollen-mass becomes transformed into a body composed of trans- 

 verse rows of cubical cells the special-parent-cells. These 

 special-parent-cells become polyhedral by unequal expansion of 

 the whole mass, and then a pollen-cell originates in each. 



The pollen -cell of Zostera appears at its origin as an obtuse- 

 angled, almost cylindrical sac, the long diameter of which is 

 three or at most four times as great as the cross diameter (figs. 

 11-13). The nucleus, which it is difficult to make visible, 

 vanishes at an early period. 



With the formation of the pollen commences a very consider- 

 able enlargement of the pollen-chambers (loculi), by vigorous 

 multiplication and expansion, in a tangental direction, of their 



* Schacht (Das Mikroskop, Berlin 1851, p. 154) assumes that only one lon- 

 gitudinal row of cells becomes converted into primary parent-cells. His figure 

 (tab. iii. fig. 8) shows the cross section of the attenuated upper end of an already 

 further developed group of such cells (a pollen-mass), where one primary 

 parent-cell, divided into two, is visible. Longitudinal and transverse sections 

 of earlier conditions show, beyond doubt, that several longitudinal rows of cells 

 of the tissue of the rudimentary anther, of unlike value (in Nageli's sense, t. e. 

 not all products of an equal number of subdivisions), take part in the forma- 

 tion of the pollen. I shall recur to this subject in another place. 



