638 SCALE-LEAVES, FOLIAGE-LEAVES, FLORAL-LEAVES. 



a further series of such transformations, especially the metamorphosis of parts of 

 foliage-leaves into tendrils, hooks, and claws, with the help of which the stem is 

 able to climb up fii-m supports towards the light, and the transformation of the 

 leaf-sheaths into mechanisms for protecting flowers against unbidden guests; the 

 consideration of these must be deferred till we deal with climbing contrivances and 

 protections for flowers in general; here, there only remains to be considered the 

 production of floating contrivances in certain marsh and water plants, and the 

 develoj)ment of special cells to assist those foliage-leaves which are unprovided 

 with scale-leaves in breaking through the soil. 



Floating arrangements occur in only a few species of plants, most noticeably in 

 the Brazilian Pontederia crassipes, and in the few species of the water-chestnut 

 {Trwpa). In both instances the leaf-stalks are swollen up into floats, and remind 

 one to some extent of the swollen utricular leaf-stalk of CepJialotus, Sarracenia, 

 and of pitcher-plants. They are distinguished from these by the fact that the 

 buoy-like swelling is quite closed, and that the partitioned interior neither contains 

 digestive organs, nor is beset with spines, &c., to hinder the exit of imprisoned 

 animals. Pontederia crassipes is not fixed in the mud beneath the water by roots, 

 but the plants float freely on the surface of the pond. It is of great importance 

 to these plants that they should have a small specific gravity, and that their leaves, 

 grouped in rosettes, which have been unfolded above the water, should offer a large 

 surface to the air, while at the same time the illumination of the green portions 

 should not be encroached upon. Both these requirements are met by the bladder- 

 like leaf-stalk, and these strange floating plants are driven Ijy tlie wind like ships 

 hither and thither over the surface of the water. 



The plants of water-chestnut are held fast to the muddy bottom under water by 

 roots, and are not adapted to floating freely. The submerged leaves are finelj"- 

 divided like a comb, and have such a small specific gi-avity that when detached 

 from the stem they immediately rise to the surface of the water. The uppermost 

 leaves lying on the surface of the water, and grouped into rosettes, have rhomboidal, 

 tough, almost leathery blades, and these also do not sink when they are isolated, 

 and therefore it is difficult to see what advantage is afforded in this instance by 

 the swollen leaf-stalk. But when in the height of summer large hea^'y fruits are 

 seen to be produced from the flowers developed amongst the leaves of these floating 

 rosettes, it then becomes evident that the floating capacity of the rosette-leaves 

 must be maintained, lest they be drawn underneath by the weight of the nuts and 

 placed in a position as unfavoui-able as could be imagined for the proper discharge 

 of their functions. 



In the subterranean buds of perennial plants the rudimentary foliage-leaves are 

 usually surrounded by scales, which function as shields and screens, and in par- 

 ticular play the part of protective organs in the work of breaking through the 

 ground. Most of these sheath-like scales, as already stated, grow up with the elon- 

 gating buds until the soil has been pierced, and their points strengthened by turgid 

 cells serve as actual earth-breakers. But in some plants which survive through 



