THE FLOWER 221 



ever, the analogy ends, for the pollen tube is not adapted, like 



the radicle of the seedling, to absorb and convey nourishment 



up to the other parts, but to feed and carry down to the ovary 



two small bodies called generative cells, 



which it discharges there, and then its work 



is done and it disappears. So it must be 



borne in mind that when we speak of the 



germination of the pollen grains, we mean 



something really very different from the 



germination of a seed. 



250. The course of the pollen tube. — 

 Cut the thinnest possible section through 

 a freshly pollinated pistil and place under 

 the microscope. Watch the pollen tubes 

 from the grains on the stigma as they de- 

 scend through the style toward the ovary. poiferg;ain'emittint 

 A pollinated strand of corn silk — which is a tube (magnified). 

 only a very much elongated style — is excellent for this pur- 

 pose. It is so thin and transparent that no section need be 

 made, and the tube can be traced as it works its way down 

 through the entire length of the threadlike style to the young 

 grain, or ovary, on the cob. The time required for the tube 

 to penetrate to the ovary varies in different flowers according 

 to the distance traversed and the rate of growth. In the 

 crocus it takes from one to three days ; in the spotted calla, 

 about five days ; and in orchids, from ten to thirty days. 

 As a rule, it occupies only a few hours. Sometimes the pis- 

 til is hollow, affording a free passage to the pollen tube; 

 in other cases, it is solid, and the growing tube eats its way 

 down, as it were, feeding on the substance of the pistil 

 as it grows. How is it in the flower you are examining ? It 

 takes a grain of pollen to fertilize each ovule, and where more 

 than one seed is produced to a carpel, as is commonly the 

 case, at least as many pollen tubes must find their way to 

 each locule of the ovary as there are ovules — provided all 

 are fertilized. 



