FUNCTIONS 



227 



That soil is best for crops which is loose enough to permit 

 air to get into it, and yet compact enough to prevent water 

 from leaving it too rapidly. Sand, for example, is excellent 

 for aeration, but poor for 

 holding water. Lack of 

 drainage, as well as too 

 great compactness of soil, 

 interferes with the proper 

 respiration of roots. 



Thus, by one means 

 or another, all living 

 cells of the plant receive 

 the oxygen which they 

 must have, and the 

 leaves are the organs 

 through which it chiefly 

 enters. When the plant 

 is not actively at work, 

 not so much oxygen is 

 needed ; plants which 

 live through the winter 

 without leaves appear to 

 get along mainly with what oxygen is already in their 

 tissues ; so also with the living 'cells in seeds. 



As has been said, we know very little about respiration. 

 We know that it appears to be principally a kind of oxida- 

 tion, which means that it is a chemical change involving 

 the use of oxygen and the giving off of energy. We know, 

 of course, that without it no work can be accomplished by 

 living things. We know that as work increases respira- 

 tion increases. This you have noticed yourself when exer- 

 cise gets you out of breath ; the body has greater need for 



Fig. 80. — Cross section of the stem of water 

 milfoil (Myriopkyllum) , a submerged plant. 

 Note the large and symmetrically placed 

 air passages. Note that the vascular 

 cylinder, indicated by v, is in the center, 

 as in roots. Note that its cells are but 

 slightly differentiated. This is common in 

 plants which grow under water ; they evi- 

 dently do not need heavy cell walls in order 

 to maintain an erect position. 



