122 The Leaf 



Lesson 4. THE MODIFICATION OF LEAVES. 



Season. Fourth week in May. 



Materials required, (a) For demonstration. 



A cactus plant in pot (see Appendix III). 



(b) For each pupil. 



Twigs of broom, cypress, gorse, and butcher s broom. 

 Pansy leaf. Part of a house leek showing leaves (one 

 of the thick-leaved sedums may be substituted). 



One spray of each of the following: common 

 vetch (Vicia sativa), ydlow vetchling (Lathyrus 

 Aphaca), Smilax (from any nurseryman). 



One of Nature's most universal laws, which as you 

 grow older you will find at work all round you, decrees 

 that every living thing shall learn to suit itself to its 

 surroundings. If it cannot do so it must make way for 

 those that can. Green plants are found growing in such 

 very different surroundings, from the wind-swept plains 

 of Siberia to tropical swamps, that they naturally take 

 many forms ; and of all the parts of which they are 

 made up no part changes as much with its surroundings 

 as does the leaf. 



The large flat surface of the leaf blade is very useful 

 to a plant as long as its roots can absorb as much water 

 as it needs to make up for the quantities given off by 

 the leaves. Some plants, however, grow under such 

 circumstances that, either because there is not much 

 water to be had or for some other reason, transpiration, 

 i.e. the giving off of water, must be lessened. This can 

 be done in several ways. Sometimes the leaves are 

 crowded closely together or have a waxy or hairy coat 



