666 LECTURE XXXVIII. 



shown why thick tendrils are unable to twine round very thin supports. On comparing 

 two 'tendrils, one of which is twined round a slender and the other round a thicker 

 support, it is obvious that in the former the proportional difference in length of the 

 outer and inner sides must be greater than in the latter. If a thick and a thin 

 tendril twined round supports of equal thickness are compared, the proportional 

 difference in length of the outer and inner sides will be greater in the case of the 

 thick one than in that of the thin one. If we now suppose the support to become 

 thinner and thinner, the proportional difference in length increases more rapidly for 

 the thick tendril than for the thin one, and it then becomes a question whether 

 the growth in length of the two sides of the tendril can or can not attain any 

 given value. As a matter of fact, the difference in length attainable by unequal 

 growth of the two sides of the tendril has its limit, as is shown by experiment. The 

 thin tendrils of Passiflora gracilis can coil closely round fine threads of silk, 

 whereas the thick tendrils of Vitis can only coil themselves round supports which 

 are at least 2-3 mm. thick. The most strongly curved Vine-tendril which I could 

 find had coiled itself tightly round a support 3-5 mm. in thickness, and this only in 

 one almost circular coil ; the average thickness of the tendril at this spot was 3 mm. 

 The concave side of one turn was therefore nearly 11 mm., the -convex outer side 

 nearly 29 mm. long, and thus the relative lengths of both sides nearly as i : 2-6. 

 If however this tendril, 3 mm. in thickness, were supposed to coil itself round a 

 support only 0^5 mm. in thickness, an almost circular coil of it would then hive a 

 length of only i-6 mm. on the concave side, and of 20-4 mm. on the convex side; 

 the two sides would then stand as i : 13, and it does not appear that such consider- 

 able differences in the length of the two sides of a tendril are possible by growth. If 

 on the contrary the problem were for a tendril which itself is only 0-5 mm. thick to 

 twine itself round a~support 0-5 mm. thick, closely and in an almost circular coil, 

 the inside of a coil need only be i-6 mm. long, and the outer side 4-7 mm., and thus' 

 the relative lengths of the inner and outer sides as i : 3. 



In order that a tendril shall cling firmly to its support, it is not sufficient that its 

 coils simply lie on the- support ; on the contrary they must press themselves closely to 

 it. That this actually takes place is shown by the fact that if tendrils are allowed to 

 coil round smooth supports which are then withdrawn, the coils at once become 

 narrower and their number increases (De Vries). This fact shows at the same 

 time that the tendril irritated by contact with a support strives to make a curvature 

 the radius of which is smaller than that of the support, provided that the support 

 is not too thin and the tendril not too thick. 



With regard to the pressure which the coils of a tendril exert on the support, 

 those cases are very instructive where thin leaves are entwined by strong tendrils, and 

 are thereby compressed and folded. 



Since the biological object of tendrils is to grasp supports— usually other 

 plants— and thus to enable the thin stems of the tendril-plant to climb, it 

 becomes a matter of primary importance to bring the tendrils into contact with 

 supports; this is usually accomplished in a wonderfully complete manner by the 

 fact that at the time when they are irritable, not only the tendrils themselves but also 

 the apex of the shoot which bears them are endowed with revolving nutation, with 

 the result that every object which can be used as a support, and which is by any 



