PHYSIOLOGY OF THE FLOWER. T15 
pollen tube and pass it on to the egg-cell. This is now fertilized, 
and is able to produce an embryo plant. The union of protoplasm 
from the pollen tube with the egg-cell is known as fertilization. 
This leads to the formation of seed and fruit, which will be con- 
sidered in Chapter X. (p. 132). 
It might be imagined that the pollen of a bisexual flower would 
generally effect the pollination of that flower. Such selfpollina- 
tion can and does occur in many cases, but many arrangements 
exist, often very elaborate, by which cross-pollination is brought 
about, z.e., the transference of pollen from one flower to another, 
either on the same or a different plant. Cross-fertilization,! 
which follows cross-pollination, results in more numerous and 
healthier seeds. 
Cross-Pollination.—(1.) It is obvious that self-pollination is 
out of the question in wnsexual flowers, where stamens and pistil 
do not occur together. Even, however, in bisexual flowers self- 
pollination is avoided in various ways. (2.) It frequently hap- 
pens that flowers are dichogamous, 7.e., the stamens and stigma 
are mature at different times. Dichogamy is exhibited under 
two different forms—(a.) Proterandry, much the commoner, when 
the stamens mature first; (b.) Proterogyny, when the opposite is 
the case. (3.) Often again herkogamy is presented. In other 
words, there are mechanical arrangements by which the pollen of 
a flower is prevented from falling upon the stigma of the same 
flower. (4.) The flowers of some plants are even se/f-sterile, 7.e., 
their pollen, if it reaches the stigma, has either no effect at all, or 
else a baneful one. What then are the agents effecting cross- 
pollination? The answer is simple—water, wind, insects, and 
birds. 
Some aquatic plants are water-pollinated. Perhaps the best 
example that can be given is Vallisneria spiralis, a plant com- 
monly found in aquaria. This possesses female flowers placed on 
long spiral stalks, by means of which they can be brought to the 
surface of the water when mature. The small male flowers, on 
the contrary, do not possess such stalks, but when their pollen is 
ripe they become detached, rise to the surface, and pollinate the 
female flowers. 
Wind-pollinated (anemophilous) flowers are characterized by 
their small size. The perianth is often absent or imperfect; 
when present, it is regular and devoid of bright coloration. 
Large size would here simply interfere with the action of the 
wind and prevent pollen from being blown freely to and from 
the flowers. Brilliant hues would be thrown away on an inani- 
1 This term is often erroneously employed to designate cross-pollination. 
