200 EXPERIMENTS WITH PLANTS 



cylindrical cells, like a lot of tubes standing on end, 

 closely packed together. These cells (p, called the pali- 

 sade cells from their resemblance to a palisade) contain 

 a good deal of leaf -green, or chlorophyll, in the form 

 of drops or granules (these are called chlorophyll 

 granules, cM. gr.) thickly scattered upon the inner sur- 

 faces of the walls of the cells. Below these is the 

 spongy tissue (sp) , composed of loosely joined cells, 

 with large air-spaces between them. Under the micro- 

 scope the air- bubbles in these spaces have a dark 

 appearance. Finally comes the lower epidermis, re- 

 sembling the upper, but provided with abundant sto- 

 mata, one of which is shown cut across at (s) . Notice 

 the opening with the two guard-cells, seen in section, 

 and, just above the opening,- a large air-chamber (a) 

 which communicates with the air-spaces of the spongy 

 tissue. The air, therefore, may penetrate through the 

 stomata to the interior of the leaf, where a great ab- 

 sorptive surface (many times greater than the external 

 surface of the leaf) is spread out. As the cells of the 

 leaf are full of liquid, this surface is constantly moist, 

 and therefore in the best condition for absorbing 

 carbon dioxide, just as the moist surfaces of the 

 lung-cells are in condition to absorb oxygen. If these 

 surfaces should become dry they would lose their 

 power of absorbing gases almost entirely. 



The leaf is, therefore, an absorbing organ, as well 

 as the root: the one absorbs air- food (carbon dioxide), 



