PHYSIOLOGY 479 



est of these carbohydrates, and the fatty oils are related to them. 

 The reserve-food found in seeds, tubers, bulbs, winter-buds, etc., is 

 very largely in the form of starch or oil, derived originally from the 

 carbohydrates formed in the leaves by photosynthesis. 



Organic Acids 



Organic acids in some form are always developed in plants. 

 They are usually the products of decomposition, but may arise syn- 

 thetically from simpler compounds. They are sometimes excretions 

 (e.g. lactic and acetic acids), but may, as in the case of succulents 

 already referred to, serve as constructive materials. Except oxalic 

 acid, which is combined in the common crystals of calcium-oxalate, 

 the acids are usually found in solution. 



Construction of Organic Compounds 



The processes of construction of organic matter are best studied in 

 the germination of seeds. The dry seed contains living protoplasm, 

 in a dormant condition, which resumes its activity as soon as the 

 proper conditions of heat and moisture are supplied. The necessary 

 materials for the first growth of the embryo plant are stored up in 

 the seed in the form of reserve-food. This, as we have already seen, 

 may be of various kinds. It may consist principally of nitrogenous 

 matter (proteids, amides, etc.), or more commonly of starch, sugar, 

 or other carbohydrates, or fatty oil. Where these are insoluble, as 

 in the case of starch and some proteids, they must be converted into 

 soluble forms before they can be assimilated. After germination 

 has fairly begun, tests show a great increase in the amount of soluble 

 substances, e.g. sugar instead of starch in, barley, asparagin in the 

 germinating seeds of Leguminosae. The food substances are now 

 in condition to be transferred, and subjected to the further meta- 

 bolic changes which are to combine the different elements into the 

 complicated living proteids. 



Ferments or Enzymes 



The conversion of the insoluble substances into soluble ones is the 

 work of the remarkable class of substances, formed by the pro- 

 toplasm, known as " Enzymes " or " Ferments." The first of these 

 to be isolated was diastase, which converts starch into sugar of some 

 kind. Some forms of diastase also dissolve cellulose. The develop- 

 ment of ferments by certain plants, especially the yeast-fungi, is 

 utilized in the processes of fermentation of beer and wine. 



