The Trees of Texas 13 
ing. It is the device by which perpetuation is insured. The chief 
problem of each organism that lives is to maintain its existence 
during life and to leave others like itself when it dies. The 
flower then is the structure upon which the perpetuation of the 
species depends and toward which all the energy of the tree is 
directed. The mightiest trees which today inhabit the earth 
began life, some of them, perhaps eight thousand years or more 
ago, when the pollen tube delivered its sperm cell to the egg cell 
in the young ovule. For in this way, and this way only, can a 
new individual originate. 
The transfer of pollen from the anther to the stigma is known 
as pollination. When the pollen is borne on one flower and the 
stigma on another, the process is known as ecross-pollination ; but 
if they are produced in the same flower it is called self or close 
pollination. Perhaps the great majority of plants favor cross- 
pollination, for there are many evidences in support of this 
theory. In many plants the flowers are incomplete; that is, 
some flowers bear only stamens while others bear only pistils, 
and in some plants these two types are borne on separate indi- 
viduals. Such plants are called dioecious. 
Cross-pollination is brought about through the agency of the 
wind, water and insects. Wind pollenized flowers are usually 
small, consisting only of the essential flower parts, while those 
which are visited by insects are usually larger, more showy and 
produce nectar and odor. The relationship which exists between 
insects and flowers is one of the most interesting chapters in 
biology. The insect and the flower have evolved together, and 
they perform for each other a mutual service. The insect visits 
the flower in order to procure food, but while there its body be- 
comes dusted with pollen, and this is transferred to other flowers 
which it then visits. The pollen of one species will grow only 
upon the stigma of the same or of a closely related species. This 
tends to keep each svecies pure. However, crossing may occur 
provided the pollen of one species is carried to the stigma of an- 
other that is closely related to it. This has led to the improve- 
ment of many of our cultivated varieties of plants as well as to 
the origin of entirely new hybrid varieties. 
Flowers are either borne singly on the stem or in clusters. 
When the flowers are solitary, they are either on short lateral 
