96 A TEXTBOOK OF BOTANY [Ch. Ill, 12 



profitable to grow Pineapples under slat shading in Florida 

 and Tobacco under thin cotton tents in Massachusetts ; 

 while some recent experiments indicate that several common 

 crops, including Potatoes, Cotton, Lettuce, and Radish 

 Hkewise do better under some shade. Corn is one plant 

 which seems to thrive l^est without an}^ shade, though it 

 is to be noted that this plant exposes not the faces but only 

 slanting surfaces of its leaves to the sun. 



The carbon choxide indispensable to food formation comes 

 from the air through the stomata ; and therefore the leaf 

 must be kept free from dirt which would clog them. Such 

 a clogging of the stomata, with consequent starvation of the 

 leaves, explains the damage now done to hedges along coun- 

 try roads by the dust thrown by automobiles, and likewise 

 the death of leaves growing near cement factories, from which 

 a very fine dust continually radiates. In minor degree 

 dust is a detriment to house plants, explaining the value 

 of an occasional spraying or washing by rain, and also the 

 following advice contained in a recent almanac, — "Cover 

 your plants kept in the living rooms with a thin cloth when 

 you sweep." Not only dust, but the floating spores of plants, 

 and also the excretions of some insects, close the stomata in 

 greenhouse plants, and necessitate the frequent scrubliings 

 which gardeners must give. Fortunately such damage is 

 minimized by the fact that most leaves have the great ma- 

 jority, or all, of their stomata upon their under surfaces. 



Water is needed by leaves for food-formation, to compensate 

 transpiration, to hold the soft tissues tensely spread, and for 

 other purposes ; and every gardener and keeper of house 

 plants knows how essential is an ample supply. In some 

 cases, however, no amount of water supplied to the roots vaW 

 comjiensate the transpiration from the leaves, because of slow 

 absorption by roots or transmission by stems. Thus are 

 explained several familiar phenomena (page 47), viz. the 

 occasional wilting of garden plants when the soil is not dry, 

 the limitation in the kinds of plants which can l)e grown in 



