640 



ECOLOGY 



just beneath the sinus prevent such tearing. Sometimes such marginal protecting 

 structures are wanting, as in the banana leaf, which consequently is shredded by 

 the winds (fig. 846). 



Leaf tendrils. Climbing organs in general will be considered under stems 

 (p. 651), but some plants (as vetches and peas, figs. 939, 943) climb by means of leaf 

 organs, the upper leaflets consisting of slender tendrils instead of blades; in Cobaea 

 the tendril ends are recurved somewhat after the fashion of grappling hooks (figs, 

 959, 960). Tendrils are irritable organs, which react by growth curvatures when 



they come in contact with a 

 solid object, and thus are en- 

 abled to coil about a support. 

 Some petioles (as in Tropaeo- 

 lum) are similarly responsive. 

 Leaf tendrils sometimes have 

 been called modified or meta- 

 morphosed leaves or leaflets, 



FIG. 939. A growing shoot of the sweet 

 pea (Lathyrus odoratus), showing leaves with 

 a pair of leaflets (/), a terminal tendril (t\ and 

 a pair of stipules (s) at the base of the petiole 



940 



FIGS. 940, 941. Bud protec- 

 tion in the sycamore (Platanus 

 occidental-Is): 940, a portion of 

 a twig, showing the swollen base 

 of a petiole; 941, a twig, as in 

 940, with enough cut away to 

 show the bud for the following 

 year (d) covered by the swollen 

 base (6) of the petiole (/>). 



a statement that is unwarranted, since there is no evidence that ancestrally they 

 were ever anything else than tendrils. 



Petioles. Attention has been called elsewhere to the chief advantage of leaf- 

 stalks or petioles, namely, the facilitation of leaf display to light through elongation 

 and change of orientation. Petioles are poorly developed in most conifers and 

 monocotyls, reaching their culmination in dicotyls, where usually they are slender, 

 elongated organs, contrasting sharply with the blades (fig. 779). Short or broad 

 petioles are of less significance in facilitating leaf display. While many petioles 

 are cylindrical, others are grooved and still others (as in the poplars) are flattened 

 laterally. Some leaves (known as phyttodes) consist only of petioles (fig. 853). 



