THE LEAF 



221 



Fig. 200. — Large air- 

 chambers in the leaf-stalk of 

 the Water-Lily, forming air- 

 canals from the leaf-blades 

 to the root. 



(c) Ways for Respiration. — The entrance of oxygen into the 

 plant-body is accomplished in the same way as that of carbonic 

 acid for assi4mlation : it enters through 

 the stomata of the leaves into the air- 

 chambers inside, and is distributed in 

 the tissues in all directions, penetrating 

 into the protoplasm of the inner cells. 

 Stems and stalks that are covered with 

 bark also have their openings, called 

 cortical pores, by which the free passage 

 of gases is secured. They appear as 



small, brown out- 

 growths scattered 

 over the surface of 

 stems (fig. 201, see 

 also stem of Shoe- 

 flower) and consist of loose cells with 

 large intercellular spaces communicating with 

 groups of cells (called medullary rays) that 

 run through the woody tissue to the centre 

 of stems. 



In marsh and water plants, which stand 

 partially in the air, e. g., the Rice- 

 plant and the Water-Lily, inter- 

 cellular air-spaces are extensively 

 developed and form connecting canals 

 (compare the leaf-stalks of the Water- 

 Lily) through which the atmospheric 

 oxygen can reach the organs growing 

 deep in the swampy soil which are 

 cut off from any communication 

 with the atmosphere. The roots of 

 Mangrove tree obtain air to breathe 

 by means of special growths thrown up above the level of the 

 water (see page 56). 



Fig. 201. — Cortical pores in a 



young stem of Elder. (Nat. size.) 



To the right a magnified pore. 



