PLANT PHYSIOLOGY 249 



that an increase in fat may be accompanied by a decrease of 

 either glucose or starch and in some cases of the hexahydroxy 

 alcohol, mannitol. In the protoplasmic cell contents fats are 

 present in the form of small globules. 



Location of Fats. — Fats, including in this term both fats 

 and oils, occur in plants mostly as reserve food in the seeds. 

 Waxes, on the other hand, occur principally in the vegetative 

 organs, where they serve as protective material against drouth 

 and cold. Fats are also found in smaller amounts in the 

 vegetative organs and may likewise be protective. In most 

 plants fats are not so abundant as carbohydrates. A few 

 exceptions to this are oil-bearing seeds such as castor-oil bean, 

 cottonseed, flaxseed and most nuts except chestnuts. 



Location of Proteins. — Protein, like fat, while occurring as 

 an essential constituent of cell protoplasm and serving as 

 energy food, is found mostly as a reserve food in seeds. It is 

 synthesized originally in the leaves, as just stated. As it is 

 found stored in seeds it is generally somewhat localized in the 

 layer just beneath the outside, as in the aleurone layer in wheat 

 grain. In the seed the protein together with the other con- 

 stituents is used as food for the developing germ. 



Source of Nitrogen. — In the metabolism of photosynthesized 

 glucose or other carbohydrate compounds into protein it is, of 

 course, only the carbon-hydrogen-oxygen portion of the protein 

 that can be supplied by the carbohydrates. The nitrogen of 

 plant protein is derived from another source. In most plants 

 the source of nitrogen is the soil, where it occurs principally in 

 the form of nitric acid salts or nitrates. In this form the nitrogen 

 is taken up by the plant roots and translocated to the cells. 

 Here it meets with the carbohydrate cell food, and the two 

 materials become metabolized into protein. In the case of 

 other forms of nitrogen which are present in the soil, e.g. am- 

 monium salts and organic nitrogen compounds (humus, etc.), 

 these are converted by bacterial action into nitric acid nitrogen. 

 The organic nitrogen compounds, consisting mostly of dead- 

 protein or protein residues, such as amino compounds, urea and 



