CH. Ill, 3] 



SYNTHESIS OF FOOD 



19 



jrUJEL practically waterproof, and thus prevents desiccation 

 of the soft leaf tissues when exposed to the sun and dry air. 

 While tightly adherent, as a rule, to the chlorenchyma and 

 veins, it can sometimes be stripped away, if started with a 

 knife, from leaves of the Lily-like kinds, while from some 

 of the flbuseleeks (or "Live for ever") it can be loosened 

 by pressure of the fingers, and later blown out, as most 

 children well know. Commonly the epidermis appears per- 

 fectly continuous and homogeneous, but in exceptional 

 cases (e.g. Wandering Jew), the hand lens will show, espe- 

 cially_on thej lr)rlp r pjde of the leaves, tiny slit-like pores En- 

 closed in greener ovals. These slits, called ST^OMATA, are always 

 present, even though rarely visible to a hand lens"." They are 

 real openings, which connect with microscopical AIR PASSAGES 

 extending everywhere through the leaf, and having greaT> 

 functional importance, as will soon appear. Also the epi- 

 dermis, while typically smooth even to shining, often bears 

 divers sorts of fine hairs or scales, called TRICHOMES, which 

 give to the leaves a grayish, woolly, or sometimes scurfy 

 appearance whereby often the clear green of the underlying 

 chlorenchyma is obscured. 



The petioles of leaves, typically cylindrical in form, consist 

 mostly of veins, with little overlying chlorenchvma ; but 

 they develop commonly some additional strengthening tissue. 

 The stipules, when present in typical form, have simply the 

 leaf structure in miniature. 



3. THE SYNTHESIS OF FOOD BY LIGHT IN LEAVES 



The prominence of leaves, in conjunction with their com- 

 parative uniformity of structure, indicates for them a very 

 fundamental function in plant life. Tbl? 1>g wp11 known jo, 

 consisUn the formation of food. rahiehrrts-eaa of the most, 



be 

 somewhaj 



All leaves are found by chemical tests to contain sugar, 

 mostly the kind called grape sugar^ which occurs dissolved 



