PHYSIOLOGY OF THE FLOWER. I I 5 



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. 



It might be imagined that the pollen of a bisexual flower would 

 generally effect the pollination of that flower. Such self-pollina- 

 tion can and does occur in many cases, but many arrangements 

 exist, often very elaborate, by which cross-pollination is brought 

 about, i.e., the transference of pollen from one flower to another, 

 either on the same or a different plant. Gross-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 unisexual 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, i.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 her~kogamy 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 self sterile, i.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. 



