72 SCIENCE PROGRESS. 



fibres and the sieve tubes, and may be present or not in 

 the pith. When the epidermis contains it, it is usually 

 in the hairs if any are present ; even root-hairs giving 

 evidence of a certain amount. Taking the members of the 

 axis, Waage found that roots as a rule contain more than 

 stems, unless the latter be rhizomes, in which it is fairly 

 abundant. Petioles and the peduncles of flowers contain 

 less than branches. In plants where the axis is highly 

 charged with it, there is generally a quantity also recog- 

 nisable in the leaves, chiefly occurring there at the edges 

 near the endings of the veins, and further in the neighbour- 

 hood of the vessels of the latter. The palisade tissue of 

 the leaf has usually more than the spongy mesophyll, and 

 the upper has more than the lower epidermis. The seed 

 as a rule contains but little, and that is only in the integu- 

 ments. 



If the disposition may be taken as any indication of 

 its being a reserve material at all, the probability is that its 

 value in the latter sense is but slight. The disposition of 

 varying amounts in the medullary rays and its frequent 

 presence in the cells of the cambium layer point possibly 

 to its supplying nutritive material for the latter. On the 

 other hand, its consistent absence from all parts of the seed 

 except the integuments seems to indicate that storage of 

 nutriment is not its main purpose. It may be that its value 

 to the meristem tissues is based upon its easily oxidisable 

 character, affording energy thereby, rather than being a 

 reserve substance. Its occurrence in the leaves in the 

 localities named suggests a formation in the mesophyll and 

 a subsequent transport to the axial regions. But against 

 the view of its value in metabolism as a reserve material 

 we have the statement that light does not affect its forma- 

 tion. It is in Waage's opinion found in the cell-sap as a 

 general rule, rather than in either protoplasm or choro- 

 plastids. It seems on the whole to be a product of 

 destructive metabolism, for it occurs in the same cells as 

 starch and sugar and may be derived from the latter by 

 abstraction of three molecules of water, C 6 H I2 6 - 3 H 2 = 

 C 6 H 6 Q 3 . It seems to resemble tannin in that it often 



