THE LEAF IN RELATION TO ITS ENVIRONMENT 



285 



FIG. 348. Monslera deliciosa. 

 The remarkable perforated leaf of this tropical Aroid. 



even larger ones have 

 been met with. Never- 

 theless, the Victoria 

 Lily is the largest of 

 floating leaves, and well 

 deserves all the praise 

 that has been lavished 

 upon it. 



Forty years ago the 

 G in i n e 11 1 G- e r in. a 11 

 botanist Hildebraiid 

 gave an a c count of 

 some interesting obser- 

 vations on the physi- 

 ology of the floating 

 leaves of Mar 8 He a 

 quadrifolia. in the Bo- 

 tanize he Zeitung. He 

 found that when a 

 plant of this species is 

 sunk beneath the sur- 

 face of the water, so 

 that all the leaves are more or less deeply covered, those leaves which are 

 fully developed at the time of immersion remain unchanged, Avhile those 

 which are not so far advanced undergo a remarkable change, the petioles 

 gradually lengthening in succession according to their position on the stem, 

 and soon over-topping those which were already formed. At first the four 

 leaflets do. not increase, but presently they begin to enlarge, and by the 

 time the surface of the water is reached they exceed in size the ordinary 

 leaves, forming a four-rayed star on the surface. "While the petioles of the 

 ordinary leaves are stiff, so that they stand erect out of the water, these 

 floating leaves are weak and flexible, like those of water-lilies, allowing the 

 leaf to maintain its position on the surface with the rise and fall of the 

 water. Their upper surface is shining and coated with wax, so that 

 the water flows off them. If immersed in deeper water, the petioles will 

 lengthen still further even to the extent of three feet. 



Before passing from water-plants, we must call attention to that delicate 

 Madagascar aquatic, the Lattice-leaf-plant (Ouvirandra fenestralis), which 

 is remarkable from the fact that the network of its leaves, instead of being- 

 filled up with tissue (parenchyma) in the ordinary way, is left open, the 

 chlorophyll in each leaf being contained in a thin layer of cells which covers 

 the strands (fig. 347). The plant is entirely submerged, and when viewed 

 from above has the appearance of a large oval piece of green net spread 

 out upon the mud in which its roots are fixed. This appearance is due. to 



