IRRITABILITY. 489 



cave surface does not elongate at all and may even become 

 shorter. 



In illustration of the relation existing between the rates of growth of 

 the two sides of the coiling portion of a tendril, and between these and 

 the rate of growth, the following observations of de Vries are cited. 



A young tendril of Curcurbita Pepo, 12 cm. long and 0^65 mm. thick, 

 was marked out into lengths of i cm., and the fourth and fifth of such 

 lengths (counted from the apex) were also marked out into millimetres. 

 The portion marked out into millimetres was then brought into contact 

 with the support, an iron wire 1*55 mm. in diameter. At the end of the 

 experiment the tendril had made i^ turns round the support, the rest of 

 it being straight. 



On measuring, it was found that 



the length of the concave surface of one complete turn of the 



tendril =4*87 mm. 



the length of the convex surface of one complete turn of the 



tendril =879 



the length of the curved portion having been, when straight =4*60 



hence, for each millimetre of original length there had been a 



growth, on the inside =0^05 



on the outside =0*9 



The mean rate of growth of the straight portions of the ten- 

 dril, above and below the curved portion, was found to 

 have been, per millimetre =0*15 



From these results it appears that the rate of growth of the concave 

 side was considerably less than the mean, whereas that of the convex 

 side was considerably greater. 



It has been observed that the twining of a tendril round a 

 support is dependent upon the thickness of the support, and 

 upon that of that tendril. Most tendrils, inasmuch as they are 

 very thin, can twine round strings, but those which are thicker 

 can only twine round a support of some thickness. This is 

 simply a mechanical necessity, inasmuch as there appears to 

 be a limit to the excess of elongation of the convex over the 

 concave side. 



For example, Sachs noticed that a tendril of a Vine had made one 

 turn round a support : the thickness of the support was 3*5 mm., and the 

 mean thickness of the coiled portion of the tendril was 3 mm., hence the 

 length of the concave side was 1 1 mm., and that of the convex side was 

 about 29 mm., the lengths of the two sides being in the proportion of 



