Ch. Ill, 3] SYNTHESIS OF FOOD 19 



It is 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 chlorenchj-ma and 

 veins, it can sometimes be stripped away, if started \\ith a 

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

 of the Houseleeks (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, e.spe- 

 cially on the under side of the leaves, tiny slit-hke pores in- 

 closed in greener ovals. These slits, called stomata, are always 

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

 real openings, which connect with microscopical .ur passages 

 extending ever^'where through the leaf, and having great 

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

 dermis, while tj-pically smooth even to shining, often bears 

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

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

 appearance whereby often the clear green of the underlying 

 chlorenchjTna is obscured. 



The petioles of leaves, typicaUy cylindrical in form, consist 

 mostly of veins, with little overhing chlorenchyma ; but 

 they develop commonly some additional strengthening tissue. 

 The stipules, when present in tj'pical form, have simply the 

 leaf structure in miniature. ^ 



.3. The Si'nthesis of Food by Light ix Leaves 



The prominence of leaves, in conjunction with their com- 

 parative uniformity of structure, indicates for them a very 

 fundamental function in plant hfe. This is well known to 

 consist m the formation of food, which, as one of the most 

 important of all processes in nature, will here be described 

 somewhat fully. 



All leaves are found by chemical tests to contain sugar, 

 mostly the kind called grape sugar, which occurs chssolved 



