ii6 The Living Plant 



diverse ways in which plants develop the same organs, applies to 

 the present matter, also, — namely, the plant makes the form of 

 food easiest chemically for it to construct, provided of course 

 there is no ecological reason for making one kind rather than 

 another. 



Class III. The Secretions, or Derivatives of Carbohydrates 



So heterogeneous are these substances in composition, proper- 

 ties, and uses, that they are held in one class by hardly any 

 stronger bond than that, while including the elements of Class II, 

 they do not belong therein. Nor is the name which I give them a 

 good one, for they include some things which are not truly secre- 

 tions, while not all of the secretions are included in this class; but 

 I can think of no better general designation. The principal 

 members are the following. 



Plant Oils. These are of two distinct kinds. First, are the fixed 

 oils, which are properly plant fats, familiar to us in the various 

 oils used in food or in medicine, notably olive oil, castor oil, cot- 

 ton seed oil. They occur rather widely scattered in plants, as 

 tiny isolated drops, scattered through the protoplasm; but they 

 accumulate in quantity in many kinds of seeds, including nuts, 

 to which they give a distinctive oily luster, and in which they act 

 very obviously as a reserve food for the use of the embryo in 

 germination. A reason why oil is stored in seeds more frequently 

 than elsewhere has been found in a linking of two facts; — first, 

 food value for food value, oil is a much lighter substance than any 

 other kind of food stored by plants; and second, the seeds storing 

 it are mostly disseminated by the wind and hence need to be kept 

 just as light in weight as possible. And with these oils as with 

 other substances, good food for plants is good food for animals 

 also, the food needs of both being closely alike. Chemically 

 these fats are rather complex, a typical formula being C57H110O6, 

 which shows that they are markedly poor in oxygen; and herein 

 Ues the reason why plenty of fresh air is needed for their assimila- 



