THE PRODUCTS OF METABOLISM 95 



Leucosin, the characteristic reserve product of the 

 Chrysophyceae and perhaps also of the Baciilariophyceae, 

 occurs as rounded gUstening granules, soluble in water, 

 which do not stain with iodine. i^^' ^"^ Nothing is known 

 of its chemical structure but it is evidently a polvsac- 



Charide.260. 270, 244a 



Cellulose is composed of /?-D-glucopyranose units linked 

 into long chains through the Cj and C4 positions. As in the 

 higher plants it is the characteristic cell wall component in 

 the Chlorophyceae and many studies, made by X-ray 

 analysis and other physical methods, of cell wall structure 

 have utilized members of this class, particularly Valonia 

 spp., as material e.g.,^^^ Cellulose is not, however, invari- 

 ably present in members of the Chlorophyceae. ^^^^ Cellu- 

 lose, as demonstrated by microchemical tests, is prominent 

 in the envelopes of Dinophyceae.^'^^ It occurs generally in the 

 Phaeophyceae and Rhodophyceae, amounting to 2 to 15 per 

 cent of the dry weight in these algae^^^ and its fundamental 

 similarity in the Phaeophyceae and in the higher plants has 

 been confirmed by chemical and X-ray examination.--^ 

 Cellulose occurs sporadically in the Myxophyceae and ap- 

 pears to be generally lacking in the Xanthophyceae, Chryso- 

 ph^^ceae and Baciilariophyceae,^^^' ^^'^ although X-ray 

 evidence shows its presence in Tribonema.^^^^ 



Algin, the characteristic intercellular substance of the 

 Phaeophyceae, of which it generally forms about 25 per 

 cent on a dry weight basis, ^^ is a calcium-magnesium salt of 

 alginic acid, a polymer of /?-D-mannuronic acid having the 

 following probable constitution:^® 



COOH 



n 



