MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 177 



on the prevailing conditions — tend to develop until a complex equilib- 

 rium is established. Since each form has different physical and chemical 

 properties the multiplicity of ways in which glucose may become the 

 basic material for a great diversity of physiological processes is evident. 

 Besides the forms in which glucose may be stored temporarily, it is utihzed 

 for the construction of the permanent framework of the plant, being 

 the substance from which many forms of carbohydrate as well as of 

 other groups of organic compounds are made. The cellulose wall is 

 secreted by each cell from its supply of hexose sugars. So also is the 

 middle lamella, which is a pentosan, a salt of pectic acid. It has been 

 suggested that fats probably are derived from carbohydrates and that 

 starch is presumably an intermediate stage in fat formation. Glucosides 

 give rise to one or more molecules of sugar on hydrolysis and it is shown 

 presently that organic acids arise from the respiration of carbohydrates. 

 Little is known concerning the seasonal quantitative variation in 

 many of these contituents. It may be said, however, that crude fiber, 

 which is composed chiefly of cellulose and lignin increases steadily with 

 age in roots and branches and that seasonal variations are insignificant 

 in comparison to this regular trend. 



Complete vegetative development depends on an adequate carbohydrate 

 supply and a plant is unable to attain its full size and ordinary shape in dark- 

 ness or particularly in the absence of red light. This is true especially of leaves. 

 If they are supplied with carbohydrates in a form in which they can be absorbed, 

 the effect of the absence of light on the size of the leaf can largely be eliminated. 

 However, different plants show more or less characteristic responses to an ab- 

 sence of light in this respect, which seems to be associated with the amount of 

 carbohydrate that tends to accumulate when the leaves are kept in darkness. 

 Bean leaves, for example, contain small amounts of carbohydrates when kept 

 in the dark; consequently the leaves do not grow. This is the case with most of 

 the fruit plants. In a certain number of plants such as wheat, starch is always 

 present in the leaves and considerable amounts of carbohydrate accumulate 

 even when the leaves are kept in darkness; these attain their usual size under 

 such conditions, though they may be narrower than the leaves of illuminated 

 plants.'''^ There are, however, other peculiarities in the form and structure of 

 plants grown in darkness which cannot be attributed in any way to the carbo- 

 hydrate supply. 



In Retaining Moisture. — Pentosans have, according to Spoehr,i" 

 the property of holding moisture. Certain pentosans develop under 

 conditions where the moisture supply is limited and furnish the plant 

 with a water-retaining mechanism which minimizes the effect of the water 

 deficiency. The moisture held by pentosans seems to be in a colloidal 

 mbcture, where it is retained tenaciously and offers resistance to desic- 

 cating agencies. This colloidally held water should be differentiated 

 from free water since it is characterized by distinct physical properties. 



