SECT. II PHYSIOLOGY 189 



spaces, or to movement in mass due to pressure. Unequal pressure is 

 set up by the warming or cooling of the air in the intercellular 

 spaces, or by movement of the part of the plant leading to changes of 

 shai)e. So far as movement in mass is concerned, the quantity of air 

 passing through depends on the amount of the pressure and the 

 diameter of the intercellular passages. Since, however, great variation 

 takes place in the dimensions of most intercellular spaces from point 

 to point in the plant, no simple law can be stated. 



While the vessels form on the whole straight tracks, closed above 

 and below, where their ends abut on parenchymatous cells, the 

 intercellular spaces form a highly branched system of air-cavities 

 communicating with one another and with the external atmosphere. 

 The communication with the outside is effected in the first instance 

 by the stomata, and also by the lenticels and organs of similar 

 function (pp. 113, 160); both diifusion and movements in mass of 

 the gases go on through these openings. 



That the intercellular spaces were in direct coramnnication with each other, and 

 also with the outer atmosphere, was rendered highly probable from anatomical 

 investigation, and has been positively demonstrated by physiological experiment. 

 It is, in fact, possible to show that air forced by moderate pressure into the inter- 

 cellular passages makes its escape througli the stomata and lenticels ; and con- 

 versely, air which could enter only through the stomata and lenticels can be drawn 

 out of the intercellular passages. The method of conducting this experiment can 

 be seen from the adjoining figure (Fig. 178). 



Intercellular air-spaces are extensively developed in water and marsh plants, 

 and occupy the greater part of the floating portions of the plant. The submerged 

 portions of water plants unprovided with stomata thus secure a special internal 

 atmosphere of their own, with which their cells maintain an active interchange of 

 gases. This internal atmospliere is in turn replenished by slow diffusion witli the 

 gases of the surrounding medium. As regards the rest of their gaseous interchange, 

 these plants are wholly dependent on processes of diffusion, since stomata, etc., are 

 wanting. Plants which possess these organs may also obtain gases by diosmosis 

 if the cuticle of their epidermis is permeable to gases. 



Sig-nificanee of the Absorbed Substances. — The materials 

 taken into a plant may be essential, unnecessary, or harmful. In any 

 particular case this can only be decided experimentally, for it would 

 lead to erroneous conclusions to assume that all substances constantly 

 present in a plant are necessary. The necessary substances may be 

 distinguished as food-materials. 



Without nourishment and without the supply of new formative 

 material neither growth nor development is possible, nor, indeed, 

 without continuous nutrition can a plant maintain itself in any given 

 stage of its development. The processes of life are connected with 

 constant changes of the living substance, both transformations and 

 excretions. Even when the supply of food ceases these processes 

 continue, so that the death of the organism from starvation must 



