HOW PLAI5TS CLIMB. 



307 



HOW PLANTS CLIMB. 

 By Rev. Prof. G. Henslow, M.A., V.M.H. 



Lecture to the Students at the Society's Gardens, June 26, 1901. 



The questions naturally arise, why do some plants climb ? and how have 

 they acquired their climbing properties ? A possible origin or cause may 

 be suggested by overcrowding ; when certain weaker plants, utilising the 

 common property of circumnutation, and acquiring a greater degree of 

 sensitiveness, became stem-twiners, leaf-climbers &c. Circumnutation 

 and sensitiveness are universal properties in plants, in varying degrees ? 

 thus, they both may be seen in germinating roots. The former is 

 exhibited by germinating stems which bend to all points of the compas? 

 as they elongate. It is well seen in the terminal shoot of a Fir-tree^, 

 which, if observed from time to time, will be noticed to have changed it,? 

 direction, until it finally straightens itself below and remains erect, the 

 apex, however, still continuing to nutate. This shows that there is a 

 certain antagonism between lignification," or the consolidation of 

 tissues, and circumnutation ; so that as soon as a stem becomes rigid by 

 strengthening itself, circumnutation tends to cease. Consequently, if 

 stems be weak when overcrowded, they might continue to circumnutate 

 when growing to greater lengths ; and perhaps such conditions might 

 be favourable for an increased sensitiveness, but of this we know 

 nothing. 



Climbing " Lianes " in tropical forests often take the forms of rope? 

 and bands which completely invest the trees in an inextricable network, 

 and a feature which has long been observed is the anomalous nature of 

 the woody stems of such climbers. They belong to several families of 

 plants, and, generally speaking, their peculiarities are characteristic of 

 their families respectively. Thus, in the Malpighiacem the tendency i? 

 to make the wood deeply lobed by excessive growth at certain points on 

 the circumference, instead of uniformly all round. Then, as the twining 

 stem becomes twisted, a result of continued growth after the stem ha?- 

 become linked to another, it now exactly resembles, and indeed acquires^ 

 the strength and flexibility of a stout cable of many strands. It i?. 

 difficult not to entertain the suspicion that Nature has been following the 

 same method as man, in making a strong rope out of what would be, when, 

 isolated, a number of weak materials. As the Lianes are necessarily 

 subjected to all sorts of strains, the cable-like form is admirably suited 

 for their requirements. 



The genus Bauhinia of the Legitminosce, and its allies, are like broadf 

 ribbons, as the stem increases only on the ends of a single diameter ; but 

 besides this flattening out, the ribbon bulges alternately first on one side, 

 then on the other, thus affording great additional strength ; while in 

 some cases, like the so-called " Monkey's Ladder " (Caulotrctus), wing- 

 like appendages are added to the sides. In others, though the stem may 

 be round, on a cross section being made, the wood is found to take the 



