26 



THE ROOT 



evaporation, and, after allowing it to grow in a dark place two or three days, 

 take it out and notice the position of the dots on the elongated root. Measure 

 the distances apart, and find out which part of the root has grown most. 



After it has grown two or three inches long, branches arise 

 upon it similar in appearance to the primary root itself, only 



thinner (Fig. 9). Instead of grow- 

 ing vertically downwards, they grow- 

 away from the primary root almost 

 at right angles to it. These lateral 

 branches lengthen in the same man- 

 ner by growth near their tips, and 

 are called secondary roots. They 

 ultimately produce tertiary roots, 

 which grow out obliquely from the 

 secondary ones, and further branch- 

 ing may go on in this manner until 

 a very extensive collection of roots 

 is obtained; the whole of which is 

 called the root-system of the plant. 



2. On careful examination of a 

 well-developed root of a seedling 

 bean, the secondary roots are seen 

 arranged in five rows along the 

 primary root, and not in irregular order, as appears at first sight. 

 They are not, however, equi-distant from each other in the rows. 

 The first to appear arise near the cotyledons, followed subse- 

 quently by others, which grow out at points nearer the tip, the 

 youngest being always nearest the apex of the primary root, 

 the older ones farther away from it. The relative age of the 

 various lateral roots can therefore be determined by examina- 

 tion of their position on the primary root. This kind of 

 sequence where the youngest parts are nearest the tip of the 

 axis on which they grow, and the older ones farther away from 

 it in regular order, is known as acropetal succession. 



Primary root of bean, 

 showing 'lateral secondary roots _; A 

 root hairs. B^ Longitudinal section 

 of a similar root, illustrating the en- 

 dogenous origin of the lateral roots. 



