W. HOPMEISTER ON THE DEVELOPMENT OP ZOSTERA. 245 



cell- walls. The pollen-cells, constantly growing longer, keep pace 

 with the enlargement of the loculi ; they thus soon attain a con- 

 siderable (fig. 14), and finally, in proportion to the permanently 

 small cross diameter, quite an enormous length (in the ripe 

 anther as much as 3 lines). They retain throughout their 

 original direction, inclined downwards from the surface of the 

 spadix. Toward the close of the increase of length they displace 

 the innermost of the three layers of cells which originally formed 

 the outer wall of each loculus. A cellulose membrane becomes 

 first distinguishable on the pollen when the long diameter is 

 about eight times the cross diameter, and at that period it is yet 

 extremely delicate. As the pollen approaches maturity it be^ 

 comes tougher, but no secretion whatever of an exine occurs ; 

 as was made known by Fritzsche. The tendency of the pollen- 

 cell to expand its membrane often outruns the growth of the 

 walls of the loculus, towards the approach of maturity. The 

 pollen-cells, restrained in their natural extension, then frequently 

 exhibit curvatures or curling of the ends and protrusions of 

 various forms (PI. VI. fig. 15 6). 



As long as the longitudinal growth of the pollen lasts, the 

 granular-mucilaginous fluid contents are uniformly diffused in 

 the cell. Toward the time of maturity we find longish cavities 

 in the mucilage, filled with a fluid of less refractive power, and 

 these are finally blended into a single cavity lying in the axis. 

 At this period we may frequently observe the active currents in 

 the dense, mucilaginous coating (containing numerous granules) 

 of the inside of the cell-wall ; more distinctly in proportion as 

 the pollen-cell is riper and the temperature of the surrounding 

 water higher. Two principal currents may be distinguished, 

 one ascending, the other descending ; one of these is ordinarily 

 stronger and of larger size. The moving mass splits up here 

 and there into several arms, sometimes becoming again confluent, 

 between which remain isolated spaces occupied by motionless 

 and more transparent fluid (fig. 15.) The pollen-cells in which 

 a particularly rapid circulation occurs are always greatly swollen 

 up (lying in fresh w^ater), and ordinarily t)urst in the course of 

 a few minutes during the observation. Change of temperature 

 in the surrounding medium has a most decided influence on the 

 circulation in the pollen-cell. When warmer or cooler water 

 is applied upon the object-holder, one of the two currents is 



