198 



CLIMBERS, PARASITES, SAPROPHYTES. 



229. In most native British tendril-climbers the tendrils are pro- 

 duced by modification of the leaves. In Peas and Vetches (Lathy- 

 rus, Vicia) they are modified leaflets in which only the mid-rib is 

 developed. In Fumitories (Fumaria and Corydalis) the leaf is cut up 

 into lobes any of which may act as a tendril. In Clematis the leaf- 

 stalk coils around the support. 



White Bryony (Bryonia dioica) belongs to the same family (Cucur- 

 bitaceae) as Cucumber, Gourd, and Vegetable Marrow, and has a thick 

 tuberous rhizome, usually branched, from which arise the long annual 

 shoots. Each tendril arises at a "node" beside a leaf, and in the 

 upper flowering part of the plant we get leaf, tendril, and inflorescence 

 arising together. In Bryonia itself, the tendril is unbranched, but in 

 Vegetable Marrow it is sometimes branched and bears leaves ; hence it 

 is likely that the upper coiling part of the Bryony tendril is a modified 

 leaf while the basal stiff part is a stem structure. The leaves are 

 divided into 5 or 7 angular toothed lobes, and the whole plant is rough 

 with small hairs. 



230. Twining Plants. In twiners the stem coils around 

 supports, and the coiling is a special response made by the 

 growing point of the stem to the stimulus of gravity. This 

 response is termed transverse geotropism, and no coiling 

 occurs if the plant is rotated, with the stem horizontal, on a 

 clinostat (Art. 190). The mode of climbing of a twining 

 stem is, therefore, totally different from the coiling of a 

 tendril, the latter being due to a special kind of irritability 

 set up by the support. 



In most cases twining only occurs around supports which 

 are vertical or nearly so, and which are not too thick, for the 

 stems of twiners have a strong tendency to ascend directly 

 upwards by the shortest possible path. 



As an example of a twiner study the Scarlet Runner. 



(a) Grow a seedling in a pot until the upper part of the shoot hangs 

 over a few Sinches. Tie the lower part of the stem to a stick placed 

 in the soil, set the pot on a sheet of paper, and record the position of 

 the tip of the shoot, either (1) by drawing lines on the paper radiating 

 from the centre of the pot, so as to show the direction in which the tip 

 of the shoot points, or (2) by using a plumb-line (a string with a weight 

 tied at one end) and marking the spot on the paper below the stem tip, 

 or (3) by fixing a sheet of glass over the plant, resting on supports, 

 and marking on it the position of the stem tip. Whichever method is 

 used, record the time when each observation is made, and find out 

 how long it takes for the stem -tip to swing round through a complete 



