154 Causes and Course of Organic Evolution 



Cork is formed beneath or by epidermal and cutinous tissue 

 as a cell layer, but as indicated in detail by Czapek (^^, 1: 

 572-580) it seems nearly related to, if not identical with, cutin. 

 The manner and degree in which it may arise amongst isolated 

 genera of a family, as in the stem of Draccena and Yucca amongst 

 Liliacege, in Agave amongst x^maryllidacese, on the 'air roots 

 of aroids like Philodendron, on the perennial roots and stems 

 of some species and yet absent from other annual species of 

 dicotyledon, are all features that we consider to be correlated 

 with environal stimuli, as calling forth or obliterating lines 

 of energy that at one time become well marked, at another 

 sink from observation. 



Passing next to the formed products or foods, the group of 

 sugars can only be glanced at. 



Glycogen is formed according to Hegler (^5; 229) in various 

 of the Schizophycese. In many fungi it has been minutely 

 studied (21^: 1 : 233). It is frequent amongst the Myxomycetes — 

 organisms formerly classed as plants, but now placed by some 

 amongst animals. Its wide distribution amongst animals under 

 the name of animal starch is well knowTi. 



Glucose or grape sugar has wide distribution amongst green 

 plants, and seems to be the first "organic" product of food 

 elaboration in sunlight. In fruits it is always associated with 

 the allied fructose. But glucose also is formed amongst higher 

 animals in the pathological state knowai as Diabetes mellitus. 

 Cane sugar or saccharose of many plants and milk sugar or 

 lactose of animals are nearly related bodies, and are frequent 

 though not invariable constituents of many plants and animals. 



Starch is one of the most typical and mdely distributed 

 reserve foods of green plants, though it seems to be absent in 

 animals. But the somewhat simpler substance glycogen rep- 

 resents it in Blue-green Algae and in animals. 



The fixed oils and fatty acids are a large and very natural 

 group common to plants and animals. A neat and continuous 

 series of glycerides of the fatty acids is that which includes 

 such well-known bodies as butyric and stearic acids These 

 illustrate well the principle of chemical substitution or addi- 

 tion — in this case the addition of CHg in the advancing series — 

 as regards molecular constitution, and, since all have been 

 definitely obtained from plant or more rarely from animal 

 substances such as the fats or tallows, we give them in con- 

 tinuous molecular relation. 



