46 



PLANT PHYSIOLOGY 



stability. Hemicellulose is found in the horny endosperm of 

 certain palm seeds {Phytelephas and Coelococcus), the so-called 

 ''plant ivory" of which buttons are made. A similar structure 

 is displayed by seeds of the date palm, coffee, peony, lupine, 

 nasturtium, and several other plants. The chemical composition 

 of the reserve cellulose varies in different plants. It contains 

 various pentosans, anhydrides of polysaccharides composed of 

 pentoses, most frequently of xylose and arabinose. These are 



Fig. 12. — Tangential section through a ramie fiber showing three chains of 

 glucose units. Dark lines indicate primary valence bonds; broken lines indicate 

 probable general direction of secondary valence forces (after Sponsler). 



designated correspondingly as xylans and arabans. Of the 

 hexosans, it contains mannan and galactan, i.e., polysaccharides 

 which on hydrolysis disintegrate into the hexoses, mannose 

 and galactose. Hydrolysis of reserve cellulose proceeds very 

 readily by enzyme action. In germinating seeds rich in hemi- 

 cellulose, it is dissolved very rapidly. 



11. Reserve Fats, Lipoids, and Phosphatides. Organic 

 Acids. — Fats and oils are found in almost all seeds. They enter 

 into the composition of the protoplasm of embryonic cells. In 

 most seeds, they are especially abundant in the embryo, repre- 

 senting reserves readily available during germination. 



