MODIFICATIONS OF LEAVES AXJJ STIPULES. 



417 



be suggested that they are developed before the wet season has fully 

 arrived, the oncoming of which then produces the green foliage. 



An analogous case is the spinescent bracts of the involucre of 

 Centanrea Calcitrapa, formed after the non-spinescent foliage has 

 developed, as the wet season is presumably closing ; for no other 

 cause is known of spinescence than drought. In a good soil and 

 plenty of moisture spinescence generally disappears, as it will if furze, 

 restharrow, or barberry, be grown in a moist air and soil. 



In a few cases the stipules act as tendrils ; that is to say, if they may 

 be so regarded, as in S?nilax and the Cucurbitaccae ; but the fibro- 

 vascular bundles in monocotyledons are all separate, and the tendrils 

 in bryony (e.g.) issue from the stem in a very similar manner, as the 

 woody fibres are anomalous in their distribution in this family. 



Lastly, in some cases the stipules secrete honey, as in Trojjaeolum 

 ciliatum (fig. 87) ; but glandular stipules are common in many families. 



One of the most interesting explanations of the action of tendrils has 

 been given by Mr. H. J. Shannon * in describing his experiments with 



Fig. 87. — Tropaeolum ciliatum with Hoxey-secreting Stipules. 



the tendril of a pumpkin : — " A pencil was hung slack against a young 

 tendril. After nine minutes the tip had curved. In ten minutes the 

 tendril tip was parallel, round the pencil, with the lower part like a staple. 

 In thirteen minutes the coil was completed and tight. The free tip, with 

 its short remaining piece of tendril, tried to make another coil. After a 

 moment or two it became evident that the curling motion had ceased 

 entirely. The shoot bearing the tendril had been gradually leaning 

 towards the pencil. The tendril, however, had exerted no ' pull,' for 

 the pencil hung vertically all the time. This movement of the shoot 

 produced a slackness in the tendril, and this slackness was now taken up 

 by the coil already formed, as it worked itself slowly forwards about the 

 circumference of the pencil, and so added this new winding material 

 to the free extremity. The tip now commenced to move forward again, 

 hugging the pencil, and by a slight upward inclination succeeded in 

 laying its flattened surface snugly and firmly against the coil already 

 formed. Once again, however, before this second coil was completed, the 

 material became exhausted and the winding motion ceased as before. 

 The stem inclined itself still further, and the length so given was again 



* Harper's Magazine, July 1907, p. 296. 



VOL. xxxiv. 



E E 



