CARBOHYDRATES 63 



stance in many plants. It has been reported as present in moulds, 

 and in ergot; in the roots of asparagus, chicory, etc.; in the leaves 

 and wood of many trees, such as the chestnut, apple, mulberry, 

 and many conifers; also as a part of the so-called " hemi-cellu- 

 loses " which are present in the seeds of many plants, notably the 

 palms, the elders, cedar, larch, etc. 



It is a white, amorphous powder, which is difficultly soluble 

 in water, is strongly dextrorotatory (specific rotatory power 

 +285), and when hydrolyzed yields mannose. 



Secalan (or carubin) is a substance which is found in the seeds 

 of barley, rye, etc., which is similar to mannan, but is optically 

 inactive. 



(D) GALACTANS 



These bear the same relation to galactose that the preceding 

 dextrosans do to their constituent hexoses. Four different galac- 

 tans have been isolated from plant tissues; they are all white, 

 amorphous solids which dissolve with difficulty in water, forming 

 gummy solutions. 



Both galactans and mannans commonly occur associated with 

 cellulose and hemi-celluloses in the seeds or other storage organs of 

 plants. They are practically indigestible by animals, as the proper 

 enzymes to hydrolyze them are not present in the digestive tract; 

 hence, they are commonly classed with the indigestible cellulose 

 as the " crude fiber " of plants which are to be used as food by 

 animals. 



PHYSIOLOGICAL USE AND BIOLOGICAL SIGNIFICANCE OF 

 CARBOHYDRATES 



If the organic compounds produced by plants be classified with 

 reference to their uses in metabolism into the three groups known, 

 respectively, as temporary foods, storage products, and perma- 

 nent structures, it is clear that the carbohydrates which have been 

 discussed in this chapter may fall into either one of the first two of 

 these classes. There can be no doubt that the first products of 

 photosynthesis, whichever ones they may be in different plants, 

 may be directly used as temporary foods, to furnish the energy 

 and material for the building up of permanent structures. Also, 

 there can be no doubt that these same carbohydrates are trans- 



