VIII] TENDRILS 111 



Cucurbitacese, Passifloracese, Sapindaceae, &c., although 

 for examples of studies in morphology where the nature 

 of an organ is determined from the place and manner of 

 its development, with full reference to other organs few 

 parts offer better exercise to the advanced student. 



Apart from the easily determined leaf-bearing tendrils 

 referred to above, those of many Passion Flowers, and such 

 Sapindaceae as Cardiospermum, Serjania and Paullinia, 

 are regarded as modified branches, and in some cases can 

 be shown to be so because they bear flower buds, so that 

 their peculiar positions at the side of a leaf, or even below 

 it, must be explained in accordance with the fact that it 

 is by no means necessary that branches should spring 

 from the axils of leaves : indeed there are plenty of well- 

 known examples to prove this also. 



Whatever the morphological nature of the tendril may 

 be, its essential characters are distinctly adaptations to the 

 climbing habit, and in the highly specialised typical ten- 

 drils we find long, thin, irritable filaments which are not 

 only carried up by the growth movements of the young 

 shoot which bears them, but which themselves also nutate 

 in small circles of their own, so that they exert a sort of 

 groping movement, and at once coil round any thin 

 support they come in contact with, and this may be 

 inclined at any angle. 



Once the end of a tendril is fixed, the coiling does not 

 cease, but extends down the free length, with one or 

 several reversals of direction, and so slings the shoot-axis 

 on a spiral spring. 



The after-effects of the continued contact on the coiled 

 tendril are often shown in increased thickening, and the 

 formation of wood-tissue, so that the stem remains 

 anchored by thick curved woody hooks, as in Solarium 

 jasminoides where the petioles are used, Uncaria (Nauclea) 



