154 CONSTRUCTION OF PLANT'S FOOD 



ianum living in the soil, and tubercle bacilli that cause and 

 inhabit the tubercles on the roots of Leguminosae and some other 

 families of plants. These occur normally in the soil in most 

 localities, and enter the roots through the root hairs and furnish 

 the stimulus for the growth of the tubercles. While these bac- 

 teria are parasitic on the green plant to the extent of utilizing 

 carbohydrate made by it, they nevertheless are useful to the 

 green plant in that they compound the free nitrogen of the air 

 into substances which the green plant can use for food. For 

 this reason the soil in which leguminous plants are grown, such 

 as peas, clover, alfalfa, etc., becomes richer in nitrogen, even 

 when the crop is harvested and only the stubble and roots are 

 left to decay and become a part of the soil. 



Restating briefly the relation of carbohydrate to nitrogenous 

 foods: carbohydrates, such as sugars and starches, are formed 

 in the chloroplasts by the use of the sun's energy directly. Later 

 some of these carbohydrates are united with salts of nitrogen to 

 form nitrogenous food substances, and from these still more 

 complex nitrogen compounds are formed with the addition of 

 elements from the salts of sulphur and phosphorus. While 

 the energy for the production of carbodydrates must be taken 

 directly from the sun (with the exception of the few bacteria 

 that produce nitrous and nitric compounds from ammonia and 

 its salts, and those that oxidize salts of iron and sulphur as above 

 stated), the energy for the compounding of nitrogenous foods 

 is taken indirectly from the sun (and hence can go on in darkness 

 and in saprophytes and parasites), by the decomposition through 

 oxidation, or otherwise, of carbohydrates and substances de- 

 rived from them. 



ILLUSTRATIVE STUDIES 



i. Study chloroplasts in a moss leaf. Mount a fresh leaf in 

 water and study it with low and high powers. Note the posi- 

 tion of the chloroplasts within a cell and count them. Put a 

 drop of chloral hydrate-iodine (see under this title in Chapter 



