LEAVES— STRUCTURE OR ANATOMY 



89 



thicker walls of the guard-cells (Fig. 1 1 4) absorb water 

 from adjacent cells, these thick walls buckle or bend and 

 part from one another at their middles on either side the 

 opening, causing the stomate to open, when the air gases 

 may he taken in and the leaf gases maj' pass out. "When 

 moisture is reduced in the leaf tissue, the guard-cells part 

 with some of their contents, the thick walls 

 straighten, and the faces of the two opposite 

 ones come together, thus closing the stomate 

 and preventing any water vapour from pass- 

 ing out. When a leaf is actively at work 

 makiJig nciv organic compojinds, the stomates 

 are usually open; when unfavourahle condi- 

 tions arise, they are usually closed. They 

 also commonly close at night, when growth 

 (or the utilizing of the new materials) is most 

 likely to be active. It is sometimes safer to 

 fumigate greenhouses and window gardens 

 at night, for the noxious vapours are less 

 likely to enter the leaf. Dust may clog or 

 cover the stomates. Rains benefit plants 

 by washing the leaves as well as by provid- 

 ing moisture to the roots. 



Lenticels. — On the young woody twigs 

 of many plants (marked in osiers, cherry, 

 birch) there are small corky spots or eleva- 

 tions known as lenticels ( Fig. 117). They mark the loca- 

 tion of some loose cork cells that function as stomates, 

 for green shoots, as well as leaves, take in and discharge 

 gases; that is, soft green twigs fiuiction as leaves. Under 

 some of these twig stomates, corky material may form 

 and the opening is torn and enlarged: the lenticels are 

 successors to the stomates. The stomates lie in the epi- 



Fig. 117. — Len- 

 t i c e l s on 

 Young Shoot 

 OF Red Osier 



(CORNUS). 



