CHAPTER YIU 



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 shows a 

 response to the earth influence (geotroinsm) and to the 

 moisture influence {hydrotropism)^ for whatever the direc- 

 tion of emergence from the seed, a curvature is developed 

 which directs the tip towards and flnally 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 



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