166 COMPOUND ORGANS OF PLANTS. 



the white water lily, Nelumbium, and Brazenia peltata, some- 

 times attain the length of from fifteen to twenty feet along the 

 shores of some of the American lakes, so as to bring their 

 flowers to the surface ; in fact, the length of the peduncle of 

 these plants appears to be wholly regulated by the depth of 

 the waters in which they are found floating. 



The essential phenomena of fecundation, consist ^n those 

 changes which take place in the pollen grains when brought 

 into contact with the stigma of the pistil, together with the 

 action of the pollenic tubes on the ovules. We have intimated 

 that poUen grains discharged from the anther-cells on the 

 stigma, are retained there by a viscid fluid, which at this time 

 most plentifully bedews the stigmatio surface. Very soon we 

 see thorn swell out, as they absorb this fluid, those which 

 are elliptical or elongated becoming almost spherical. At the 

 end of a certain time, consisting of a few hours for some species, 

 and many days for others, the thin and highly extensible iutine 

 or inner coat of the pollen grain is seen protruding in the form 

 of a tubular or vermiform appendage. 



The mode of dehiscence of the pollen grains is always deter- 

 mined by their structure. Those which present pores, grooves, 

 or folds on their exterior surface, usually emit their tubes at 

 these points. The number of tubes emitted from pollen grains 

 is very variable ; sometimes we see only one, and occasionally 

 two or three, as in the triangular pollen of the evening prim- 

 rose (CEnothera) Fig. 70. Amioi was able to detect from 

 twenty-six to thirty tubes which were protruded from the same 

 cell. The number of tubes must necessarily bear some relation 

 to the number of pores when these exist, and we know that 

 they are sometimes very numerous. The pollen tubes are 

 fiUed with a fecundating fluid termed fovillse, and it is easy to 



