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. ae 
A pollinated strand of corn silk — which is 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, 
