44 



BOTANY 



PART I 



of many insectivoi'ous plants may have commenced their development 

 in much the same way. The leaves of Nepenthes robusta (Fig. 4G), for 

 example, in the course of -adaptation to the performance of their 

 special function, have acquired the form of a pitcher with a lid which 

 is closed in young leaves, but eventually 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). 



Fig. -17. — Utricularia vulgaris. A, Part of leaf witli .several bladders (x 2). B, Single pinnule of 

 leaf with bladder (x 0). C (after Goebel), Longitudinal section of a bladder (x 28); v, 

 valve ; a, wall of bladder. 



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

 mits 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 shoots of the same name these are elongated 

 and enable the bud produced at the end to develop at a distance from 

 the parent plant. On this account Camjitosorus rhizophylhis, an American 

 Fern, is commonly known as the Walking Fern. Such leaf-runners 

 usually lose their pinnae and are reduced to the leaf stalk. A 

 particularly striking appearance is presented in those cases in which* 

 the first leaf of the bud in its turn forms a leaf-runner, so that a 

 sympodium of runners results (Asplenium obiiisifolivm, A. Mannit). 



A metamorphosis of the whole leaf lamina, or a part of it, into 



