LEAVES AS ABSORBING ORGANS 69 



layer of cells forming the epidermis is interrupted at in- 

 tervals with pores known as stomata which have guard 

 cells that open and close. Generally speaking, there are 

 more stomata in the epidermis that covers the under side 

 of leaves, though pendant leaves, or vertically erect 

 leaves, having equal exposure to light on both sides, 

 have a more nearly equal number of stomata on the two 

 sides. The flat, thin expanse which we call a leaf has 

 a framework of " veins " or " nerves " which are the 

 supporting and conducting tissues. These are the con- 

 tinuation and extension without interruption in the 

 leaves of the supporting and conducting tissues of the 

 stem, branches, and twigs. 



• Between the epidermis and the framework is the 

 mesophyll, the cells of which contain chlorophyll. The 

 uppermost layer of mesophyll cells may be more or less 

 brick-shaped with relatively small spaces between them. 

 This layer is called the palisade layer. Between it and 

 the epidermis covering the lower surface there is a more 

 or less spongy tissue, the cells of which contain many 

 chlorophyll grains, and between the cells are air-spaces 

 which are continuous throughout the mass of the leaf. 

 Into and through and out of these spaces carbon dioxid, 

 oxygen, and water vapor diffuse. It is from these inter- 

 cellular air-spaces that carbon dioxid and oxygen enter 

 the cell, dissolving in the water of the cell-walls and 

 diffusing at rates varying with the difference in concen- 

 tration of these gases inside and outside the cell. 



Leaves as Absorbing Organs. — Leaves, then, are 

 thin absorbing organs spread out in the air. They ab- 

 sorb both light and certain gaseous constituents of the 

 atmosphere. Because of the pigments which they con- 

 tain they absorb only certain rays of light. They absorb 

 only certain constituents of the atmosphere, namely, 

 carbon dioxid and oxygen, because they use only these 

 two. They absorb COo only during the hours of photo- 

 synthetic activity, during the hours of warm daylight. 



