308 JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



form of a cross, with large medullary rays of softer tissue intersecting it. 

 However anomalous the wood may be, certain common features prevail, 

 in that there is always a feeble lignification of the wood-fibres, with 

 which are associated very many and large vessels, or long tubes. The 

 usefulness of these two features is, in the former, excessive flexibility, 

 and in the latter an easy means for water to be conveyed with great 

 rapidity to the enormous lengths over which it must necessarily run to 

 reach the foliage. 



English climbers are mostly herbaceous. Clematis Vitalha, or the 

 Traveller's Joy, and Honeysuckle, almost alone possessing anything of 

 the nature of a woody stem ; but these, as also a six-year- old stem of 

 the Bittersweet (Solanum Dulcamara), exhibit just the same features in 

 being excessively flexible and provided with many vessels. 



Climbing plants may be grouped as follows : — 



1. Those which climb by means of their stems ; twiners, as they are 

 called. 



2. By branches. 



3. By leaves ; of these they may climb (1) by petioles, (2) by leaf- 

 apices, (3) by midribs modified as tendrils. 



4. By floral axes, as peduncles and pedicels. 



5. By means of hooks ; such may be (1) abortive branches, (2) leaf- 

 lets, (3) peduncles, (4) cortical and epidermal prickles. 



6. Lastly, there are aerial root-climbers. 



As a great variety of these are described by Mr. Darwin in his work 

 on Climhing Plants, it is unnecessary to give details here ; but what one 

 wishes to suggest, if possible, is the cause of the production of these 

 different structures. 



The old idea was that they, as indeed all adaptive structures in both 

 the animal and vegetable kingdoms, were " designed," i.e. in anticipation 

 of their use. This view cannot now be entertained, and we must look to 

 the reverse process for their origins. That is to say, instead of a climbing 

 organ being made before the plant climbed, we now regard it as a result, 

 if not of having actually climbed, at least of having come in contact 

 with some foreign body. To show the tenability of this view, we have 

 first to observe the extreme sensitiveness to contact which exists in plants. 

 Thus, a loop of thread weighing a quarter of a grain is sufficient to cause 

 the petiole of Clematis montana to bend ; and when an organ has caught 

 a foreign body and remains in contact Tsith it, this sensitiveness compels 

 it to develop extra tissue to an extraordinary degree. Thus, the petiole 

 of Solanum jasminoides has three fibro-vascular cords on the lower side ; 

 but, after clasping, the wood forms a complete zone as in an ordinary 

 stem. Similarly, hooks, if they catch anything, thicken and enlarge out 

 of all proportion to the size attained by non- clasping individuals. Thus 

 one arrives at the conclusion that sensitiveness causes a climbing organ 

 first to twist round its support, and secondly to thicken. From these 

 facts one deduces the origin of the form of the organ, say the tendrils of 

 the Pea. It consists of the midribs only, which are now highly sensitive. 

 As a leaflet acquires this property, so in compensation the power to make 

 the flat blade ceases. An intermediate condition is seen in Corydalis 

 claviculata, described and figured by Mr. Darwin, in which the leaflets 



