Leaves and Their Work 14-7 



without being able to extract any from the air to feed 

 upon, while the other had obtained food enough for grow- 

 ing and breathing, too. 



There is some difference in the breathing of the leaves 

 and the blossoms of a plant. The blossoms breathe faster 

 than the leaves, especially when they are just opening; 

 and they are therefore slightly warmer — in some cases, 

 indeed, so warm that the heat may actually be felt! And 

 the same is true of sprouting seeds when there are many 

 together. The sprouting barley, for instance, from which 

 malt is made, gives off quite a high degree of heat. 



As the plant, like the animal, must breathe in order to 

 live, it must needs have a constant supply of air for breath- 

 ing, as well as feeding, purposes. And this air is neces- 

 sary, more or less, not only for leaves and blossoms, but 

 for all parts, including stems and roots. It breathes 

 through all, though chiefly by its leaves and blossoms and 

 the roots are always breathing out carbon dioxide into the 

 soil. If the soil be too close, or baked on the top, the 

 roots cannot get enough air to breathe properly, and 

 the plant is sickly. And the same thing happens when 

 soil is piled up round the stem, for the air is then kept 

 away both from it and from the roots, and many a tree 

 has been killed in consequence. 



QUESTIONS FOR REVIEW 



1. Name some of the most common nitrogenous substances. 

 Why are they so called? 



2. W^hy is it rash to conclude that they are equally 

 nutritious? 



3. What is the most nutritious part of the plant, and when? 

 Illustrate. 



