188 PHOTOSYNTHESIS 



possibility is suggested that it is formed by the condensation of glycolHc 

 aldehyde and formaldehyde ; both in the hydrated form : 



H CH,OH 



CHoOH O I 



I I CH.OH 



H — C — fOH'~H;0 — C — H > I 



I ■ I 9 



OH O I 



H CHoOH 



The substance crystallizes in needles and melts at 148"^ ; it is readily 

 soluble in cold water and hot glacial acetic acid, but is insoluble in most 

 organic solvents. It does not reduce Fehling's solution and forms no 

 phenylosazone, and is not easily hydrolyzed. It forms a tribenzoyl de- 

 rivative melting at 52-53°. 



b. Tetroses. 



Tetroses do not appear in plant leaves. The alcohol erythretol has 

 been found as an ester of orsellic acid in erythrin, CooHooOio . l^^HoO, in 

 certain algae and lichens. 



Digitoxosc, CgHioOi, has been obtained as a product of hydrolysis 

 of the glucoside digitoxin which is found in the leaves of digitalis. From 

 the investigations of Kiliani it has the constitution CH3 — (CHOH)3 — 

 CH2 — CH : O, and is a partially reduced methyl pentose. Digitoxose 

 crystallizes from alcohol on the addition of several volumes of ether in 

 prism and plates, melting at 101^ and (a)o = + 46". It is easily soluble 

 in water, alcohol and acetone. It forms one oxime. melting at 102°, 

 soluble in water and alcohol but not in ether ; it does not form a crystalline 

 phenylhydrazone or phenylosazone. The cyanhydrine, on hydrolysis forms 

 an acid, the lactone of which crystallizes in needles melting at 158° and 

 possessing the formula CyHioOg.^' 



Apiose, C5H10O5. has been obtained by the hydrolysis of the glucoside 

 apiin present in the leaves of parsley, celery, etc. According to Vonge- 

 richten ^* it is a (3-oxymethyltetrose : 



CH.OH 



>C(OH).CH(OH).CH:0 

 CHoOH 



It has not been obtained in crystalline form, has a slight positive rotation 

 and is not fermentable. On reduction with phosphorous and hydriodic 

 acid it yields isovaleric acid which indicates that there is a forked chain 

 in the molecule. Apiose forms a phenylosazone, melting at 155-157° 

 crystallized from alcohol, and is soluble in acetone and somewhat soluble 



"Kiliani, Arch. Pharm., 233, 299 (1895); 234. 273 (1896); 235, 425 (1897); 

 237. 446 (1899) ; 251. 562 (1913) ; 254, 255 (1916). 



" Vongerichten, E., Ann. Cheni., 318, 121 (1901); 321, 71 (1902); Ber. chem. 

 Gcs., 39, 235 (1906). 



