364 A TEXT-BOOK OF BOTANY 



on the stem, and the successive pairs alternate with one another, 

 forming the decussate arrangement of leaves (Figs. 180, 181, 184). 



Modified Leaves. Leaves are variously modified and serve 

 for other purposes than those already described. They may be 

 fleshy in character and serve as storehouses for nutritive material, 

 as the seed-leaves of the oak, or they may serve for the stor- 

 age of water, as in Agave, Aloe and succulents. ' In some instances, 

 particularly when situated near the flowers, they lose their green 

 color, as in the dogwood, skunk cabbage and others. In other 

 cases they are modified so that they serve as a trap for insects, 

 as in Dionaa, Sarracenia and Drosera (Figs. 208, 209). The peti- 

 ole may become enlarged and perform the functions of the leaf, as 

 in the Acacias of Australia ; or it may become bladder-like and 

 serve as a means for floating the plant, as in the water hyacinth. 

 The stipules may likewise be modified, becoming leaf -like, as in the 

 pansy (Fig. 201 ) ; or metamorphosed into thorns, as in the locust ; 

 or clasping, as in Polygonum. In some cases the leaves are very 

 much reduced, their functions being performed by the stem, as in 

 Cactaceae, or even by the roots, as in some orchids which have 

 assimilating roots. 



Prefoliation or vernation is the disposition of leaves in the 

 bud. The terms used to describe the folding of the leaves in the 

 bud are derived from an examination of transverse sections of 

 the bud. The following are some of the terms which are em- 

 ployed: CONDUPLICATE, when the lamina of the leaf is folded 

 lengthwise along the midrib so that the two halves of the upper 

 surface lie together, as in the Magnoliacecc; PLICATE or plaited, 

 when the lamina is folded along the veins, like a closed fan, as in 

 the maples; CONVOLUTE, when rolled lengthwise and forming a 

 coil in cross section, as in the Rosaceoc; INVOLUTE, when both mar- 

 gins are inrolled lengthwise on the upper surface, as in the violets ; 

 REVOLUTE, when both margins are inrolled lengthwise on the lower 

 surface, as in Azalea. 



In addition, there are several terms used which are derived 

 from the appearance of the bud, as RECLINATE or inflexed, when 

 the upper part is bent on the lower, as in Liriodendron ; and 

 CIRCINATE, when the upper part is coiled on the lower so that the 

 tip of the leaf is in the center of the coil, as in the ferns. 



