54 THE CARBOHYDRATE ECONOMY OF CACTI. 



was discovered that an impure culture of yeast had been used for the 

 fermentation. ) 



To summarize, then, the observations of the seasonal variations in carbo- 

 hydrate-content : 



Low water-content and high temperatures are associated with: (1) in- 

 crease of polysaccharides ; (2) decrease of monosaccharides ; (3') increase 

 of pentosans. 



High water-content and lower temperatures are associated with: (1) 

 decrease of polysaccharides ; (2) increase of monosaccharides ; (3) decrease 

 of pentosans. 



It remains, then, to determine experimentally whether under controlled 

 conditions such a shifting of the relative amounts of the various sugars 

 actually takes place, and to what extent the two factors, water-content and 

 temperature, contribute to such an action. Before turning to these experi- 

 ments, it may be of some value to consider briefly some other observations 

 pertinent to the general problem. 



During March the new joints develop ; these grow quite rapidly, so that 

 within about one month they have attained their full size of 100 to 125 cm. 

 Apparently the young joints become autonomous very early in their develop- 

 ment ; when cut from the plant with but a very small portion of the parent 

 joint, and placed with the base in tap-water, the young joints grow to full 

 size and develop normally. Comparative analyses of young and parent 

 joints are given in table 17. The first young joints (March 27, 1916) were 

 2 to 4 cm., about 12 days old. 



The young joints are very high in total sugars compared with the parent 

 joints. Although the young joints have attained almost their full size by 

 April 16, the percentage of cellulose has not increased and the joints are 

 very tender, while in the old joints the cellulose is considerably higher, due 

 to the development of walls and vessels. The inorganic constituents increase 

 rapidly in the young joints. It is a striking fact that although the young 

 joints have a higher total sugar-content than the parent joints, both the 

 actual percentages and the proportional values of the monosaccharides are 

 lower in the former. This is probably due to the high respiratory activity 

 of the young joints. It is evident that the amount of pentose sugar is con- 

 siderable, even in the very early stages of development, although the propor- 

 tion to the total sugars is somewhat higher in the old joints. 



It has commonly been affirmed that the pentosans accumulate in the older 

 portions of a plant. This contention is based upon results obtained from 

 the older methods of analyses, in 'vhich the plant material was treated with 

 hydrochloric acid of such concentrations as to affect also the cellulose. As 

 by this treatment cellulose also yields furfural, the formation of which is 

 taken as a measure of the pentosan content, it is not surprising that the 

 older portions of a plant, which are very generally richer in cellulose, should 

 give results indicating large quantities of pentosans. That the older parts 

 of plants are not always richer in pentosans than the portions more recently 

 formed is shown by results of analyses of the following two sets of joints. 



