MORPHOLOGY 



43 



Forget-me-not (ifyvstts) are good examples of such dorsiventral shoot.-. Tlie lint- 

 joining successive leaves in such case is, at the best, but a zigzag. On the 

 ground of such observations as these it may be concluded that the actual leaf- 

 arrangements represent adaptations to definite conditions of life, and that with 

 alterations in the latter other arrangements must arise. 



The Metamorphosis of Foliage Leaves. A striking modification 

 of foliage leaves is seen in peltate leaves, in which the petioles are 

 attached to the lower surfaces somewhat within the margin, as in 

 the leaves of the Indian 

 Cress (Tropaeolum majus, 

 Fig. 191). In the process 

 of their development 

 the young leaf -blades, in 

 this case, grow not only 

 in the same direction as 

 the petioles, as a prolonga- 

 tion of them, but also hori- 

 zontally in front of them. 

 The tubular leaves of many 

 insectivorous plants may 

 have commenced their de- 

 velopment in much the 

 same way. The leaves of 

 'Nepenthes robusta (Fig. 46), 

 for example, in the course 

 of adaptation to the per- 

 formance of their special 

 function, have acquired the 

 form of a pitcher with a 

 lid which is closed in 

 young leaves, but eventu- 

 ally opens. The pitcher, as 

 GOEBEL has shown, arises 

 as a modification of the 

 leaf-blade. At the same 

 time the leaf-base becomes 

 expanded into a leaf-like 

 body, while the petiole 

 between the two parts sometimes fulfils the office of a tendril. 

 By a similar metamorphosis of its leaflets, bladder-like cavities 

 are developed on the submerged leaves of Utricularia (Fig. 47). 

 The entrance to each bladder is fitted with a small valve which 

 permits the ingress but not the egress of small water-animals. 

 While such leaves display a progressive metamorphosis the 

 modification may be of the nature of a reduction, as is the case 

 in many Ferns, which form leaf-runners. Like the modified 



I-'K;. 4ii. Xi-penthes robuxtc. (.', iial. sixr.) 



