132 GROWTH. [CH. VI 



(168) Respiration necessary. 



Pick out 12 germinating beans with roots 20-25 

 mm. in length. Having gently dried the roots by 

 stroking them with the torn, feathered edge of a piece 

 of filter-paper, mark each at 10 mm. from the tip, by 

 painting a transverse line with good Indian ink\ When 

 the ink is dry, place the seeds in water for a few minutes 

 to allow the roots to recover from the effects of the dry 

 air of the room. Then impale the seeds on "blanket 

 pins," fixing 6 in one jar (A), 6 in another (B). Fill A 

 with water so that the cotyledons are completely covered, 

 while B is only half filled and the roots allowed to grow 

 in damp air. After 24 hours remeasure : the beans in A 

 will have grown very slightly in comparison with those 

 in B. It is a remarkable fact that the roots of the bean 

 are not able to obtain enough air from the water, but 

 are dependent on their cotyledons. For another experi- 

 ment on this point see experiment 185. 



(169) Full tv/rgescence necessary. 



Proceed as in exp. 168, but let the water in jar A be 

 sufficient to cover the roots and just to touch the hilums 

 of the seeds: in jar B place a similar amount of 1 p.c. 

 NaCl solution. Measure again after 12 or 18 hours, when 

 the retarding effect of the salt solution should be obvious. 



^ Black photographic varnish may also be used, but the marks oft^n 

 become loose from water getting under the crust of varnish. On the 

 other hand Indian ink becomes faint in colour in water. Pfeffer {Druck 

 vmd Arbeitsleistung &e. 1893, p. 294) recommends Bormann's " unaus- 

 loschbare schwarze Tusohe " which must not be used in too concentrated 

 a state. 



