70 



The Living Plant 



structures with particular functions not belonging in any of the 

 definite categories above mentioned. Thus, the bladdery air- 

 filled floats which keep the Water Hyacinth resting so lightly 

 on the water are petioles; the wing which ensures the carriage 

 of the Linden seeds is a leaf-blade (figure 157) ; the indurated hooks 

 by which some tropical vines do their climbing are stipules; while 

 the reduced or rudimentary leaves which we call bracts often 

 also possess functions of a minor sort. 



Substitution foliage. Finally, we must take notice of another curi- 

 ous transformation in function and structure found in all parts 

 other than the leaf-blade, namely, they may be- 

 come transformed into foliage, either in aid of the 

 blade, or its replacement. Thus, in some kinds, 

 the blade is greatly reduced or missing, and the 

 petiole is flattened and thin and acts as the foliage, 

 e. g. in the Australian Acacias (figure 21), and 

 some kinds of Oxalis. In a good many plants 

 the stipules are sufficiently big to render appreci- 

 able aid to the leaf-blade. In Lathyrus Aphaca 

 (figure 18) they form all of the foliage there is, 

 while in the common Bedstraw or Galium, they 

 are as large as the leaves and so like them as 

 Tened petiole serv- commouly to be thought additional leaves helping 

 ing as foliage ^q niake up a whorl. In a great many plants, 



(the blades being /^ _ ^ J i ) 



insignificant), in and especially those found in dry places, the leaves 



an Australian , ,, , i ii c j- 



Acacia; one-half Dccome Very Small or are absent, and the function 

 natural size. ^£ foliage is performed by the stem, which either 



remains smooth and round, or becomes fluted by the presence of 

 vertical green ribs, or becomes flattened in various degrees, all 

 three conditions of which are found in the family of Cactuses. In 

 some cases the stem is flattened as thin as a leaf, while still dis- 

 playing the nodes distinctive of the stem, as in the Muehlenbeckia 

 of our greenhouses (figure 22) ; but in other cases no nodes appear, 

 and the stem assumes a form and general aspect so leaf -like that 



