27 



selects and describes types, and we must expect to find a 

 number of forms intermediate between the types. For each 

 intermediate form we employ the term which seems to describe 

 it most correctly, and in many cases a combination of terms, 

 such as ovate-lanceolate, oblong -lanceolate, and so on, are found 

 to be most suitable. The student should collect leaves of 

 various plants and endeavour to frame terse descriptions of 

 the same, so that another person from the descriptions may 

 be able to picture to himself the exact form of the leaves in 

 question. On one and the same plant all the leaves are not 

 exacth r the same size or shape, and hence, when describing the 

 leaves of the plant, the extremes must be given, such as 

 : ' varying from ovate to elliptic, length of blade 4-6 inches, 

 breadth 2-3 inches." 



24. When the size and shape of Polymorphic 



the leaves on one and the same plant vary very consider- Leayes - 

 ably the plant is said to be heterophyllous and the leaves are 

 said to be polymorphic. This is the case in many climbing 

 plant ssuch as Ficus pumila, the Ivy and others, see Figs. 1 

 and 2, Plate VI. 



To understand this phenomenon we must remember that 

 the function of the green leaves, viz. that of manufacturing 

 food for the plant, can only be properly performed when the 

 green leaf-blades are exposed to the sunlight and cannot be 

 carried out in the dark, and that the amount of food which can 

 be manufactured depends on the amount of green surface 

 exposed to the light. Creeping stems which are often found in 

 shady places on the ground, or on the trunks of other trees, 

 must make the most of the faint light at their disposal and 

 must keep as large a surface of green leaf-tissue exposed to 

 the light as possible. We thus find beautiful arrangements of 

 leaves of very various size and shape, small leaves occupying 

 the spaces between the large ones and the lobes and segments 

 of some leaves fitting into the spaces between similar lobes 

 and segments of adjoining leaves. Each leaf is thus arranged 

 so as to shade its neighbours as little as possible and no light 

 is allowed to pass through on to the substratum below with- 

 out being utilised. 



Such arrangements of leaves are called leaf-mosaics. When 

 such creeping stems send up erect serial stems standing away 

 from the substratum covered by the leaf-mosaic and which 

 ultimately give rise to flowers and fruit, the leaves borne by such 

 stems usually differ considerably from those on the creeping stems 

 and are more uniform in size and shape, see Fig. 2, Plate VI. 



