CHAPTER VIII 
AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS 
FOR the purpose of summarizing the general life-rela- 
tions detailed in the preceding chapters, it will be useful to 
apply them in the case of a single plant. Taking a com- 
mon seed-plant as an illustration, and following its history 
from the germination of the seed, certain general facts 
become evident in its relations to the external world. 
94. Germination of the seed. The most obvious needs of 
the seed for germination are certain amounts of moisture 
and heat. In order to secure these to the best advantage, 
the seed is usually very definitely related to the soil, either 
upon it and covered by moisture and heat-retaining debris, 
or embedded in it. Along with the demand for heat and 
moisture is one for air (supplying oxygen), which is essen- 
tial to life. The relation which germinating seeds need, 
therefore, is one which not only secures moisture and heat 
advantageously, but permits a free circulation of air. 
95. Direction of the root. The first part of the young 
plantlet to emerge from the seed is the tip of the axis 
which is to develop the root system. It at once appears to 
be very sensitive to the earth influence (geotropism) and 
to moisture influence (liydrotropism], for whatever the 
direction of emergence from the seed, a curvature is devel- 
oped which directs the tip towards and finally into the soil 
(see Fig. 143). When the soil is penetrated the primary 
root may continue to grow vigorously downward, showing 
a strong geotropic tendency, and forming what is known 
as the tap-root, from which lateral roots arise, which are 
138 
