144 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 

 dr}^, 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. Or the total length of the radicle may be measured 

 from a pin-hole, blackened with ink, in the triangular flap 

 of the testa. 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 growth of the roots 

 in A will be much smaller than (e.g. one-fifth of) that 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 experiment 

 on this point see experiment 185. 



(169) Effect of salt solution^. 



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

 sufficient to cover the roots and just to touch the hilums 



1 Sachs, iu bis Arbeiten, i. p. 408. 



2 Black photographic varnish may also be used, but the marks often 

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

 other hand Indian ink becomes faint in colour in water. We have with 

 advantage followed Pfeffer {Driick- und Arheitsleistung &c. 1893, p. 294) in 

 the use of Bormann's " unausloschbare schwarze Tusche." 



3 See Stange, Bot. Zeitung, 1892, p. 342. 



