58 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



THE PRIMARY SUGAR OF PHOTOSYNTHESIS IN THE BROWN 



AND RED ALG^:. 



Notwithstanding the close resemblance between the 

 ultimate forms in which the products of photosynthesis 

 are stored in the red and brown algae and in the higher 

 plants, there is as yet no evidence of the occurrence of 

 sucrose in the two former classes. Glucose and fructose 

 are, however, certainly present in the Phseophycese, and 

 though their identification in the Rhodophyceae has not 

 as yet been confirmed, it is very likely that it will be, when 

 due regard is paid to the variations which may be expected 

 to take place owing to differences in illumination, tempera- 

 ture, and age, of the plants. Thus, up to the present the 

 marine algae afford no support to the view that sucrose is 

 the first sugar to be formed in photosynthesis ; nevertheless, 

 that which regards this position as being occupied by a 

 hexose is by no means surely established. Much work 

 remains yet to be done in this domain of physiology before 

 any definite conclusions can be reached 



OCCURRENCE OF MANNITOL. 



Kylin and Segers-Laureys (1913) also confirmed the 

 presence of the polyhydric alcohol mannitol in the brown 

 algae. It is found to form a large proportion of the white 

 incrustation appearing on Laminarias when dried. This 

 substance has also been reported as occurring in many 

 other plants for example, Syringa vulgaris, Fraxinus 

 excelsior, Polypodium vulgare, Agaricus campestris. Among 

 mushrooms it is very widely distributed, but Bourquelot 

 (1903) has shown that its origin is the trisaccharide 

 trehalose. The latter is hydrolyzed to (/-glucose by tre- 

 halase, and then by an ill-understood biological reduction 

 this substance gives mannitol. 



