220 



THE STUDY OF LEAVES. 



ALTERNATE 



A demonstration of mosaics out of doors upon trees is 

 desirable. Branches should be studied in detail ; in this 

 way we may explain, e.g. the unequal lobing of the leaf of 

 the lime or elm. Spread out a branch of a lime tree and 

 the effect of the unequal lobing as a factor in the mosaic 

 arrangement will be clearly seen. 



The case of linear leaves of grasses should be interpreted 

 with reference to their crowded situations. 



A set of drawings of 

 good examples of mosaics 

 should be framed by the 

 pupils, e.g. the rosette mo- 

 saics of daisy and plantain, 

 the mosaic of the nettle 

 (view the nettle plant from 

 above), sycamore, lime, elm, 

 ivy, etc. 



Whilst in the main the 

 shape is " adapted " with 

 reference to the light, other 

 influences may of course 

 affect the leaf. For exam- 

 ple, the pointed tip of many 

 leaves is suited to the carry- 

 ing off of rain. The surface 

 gloss upon evergreens due 

 to the presence of a thick 

 cuticle is well adapted to 

 causing the snow to slide off 

 the leaves, avoiding break- 

 age of branches ; a specially thick cuticle is also of service 

 in checking too rapid transpiration of moisture. 



How are leaves in general lighter on the under side than 

 on the upper ? Are there any leaves which are light green 

 above as well as below ? Without the aid of a microscope 

 we cannot in any very direct way demonstrate the cause of 

 most leaves being darker on the upper than on the under 

 side. If the teacher possesses one he may do this very 

 easily. Take a portion of a fairly strong leaf, e.g. ivy, 



DECUSSATE 

 CYCLIC 



WHORLED 



Fig. 88. Types of leaf arrangement 

 on stem. 



