304 Thr V(irh<,lni(lnitci< of the Maiir/olff Lraf 



The fact that during tlie early stages of growth, when leaf formation 



is the principal function of the plant and the roots are merely small 



tap-roots, the cane sugar in the leaf is always in large excess of the 



I s 

 hexoses (the ratio '-^ varying between 0-13 and 0-71) also points strongly 

 c>s. 



to the cane sugar being a primary product and the hexoses as being 



formed by inversion from this sugar. We show in a separate paper 



(p. 329) that the proportions of dextrose and laevulose in the mixture 



of reducing sugars is always nearly that corresponding with this view. 



In September and October, when the call upon the cane sugar in the 



leaf is actually greatest for purposes of storage in the root, the ratio 



of hexoses to saccharose in the leaf rises; it is 0-94-1-59 in September 



and l-20~l-9.5 in October. The relative position of the saccharose and 



hexose curves thus entirely changes as the function changes; in August 



the invert sugar curve was much below the cane sugar curve, but in 



September the positions were reversed. In October, the curve of hexoses 



was still further above the curve of cane sugar. 



The facts we have brought forward as to the translocation of the sugars 



in the mid-ribs and stalks to our mind outweighs all the other arguments 



which have hitherto been advanced to show that dextrose and laevulose 



are precursors of the cane sugar in the leaf. AVe consider that in spite 



of the fact that it would be simpler, on theoretical or a priori grounds, 



to consider the hexoses as formed before the more complicated disac- 



charide, saccharose, the facts we have brought forward are better in 



accord with Brown and Morris' view that the cane sugar is the primary 



product of synthesis. It would seem, indeed, that plant leaves in general 



possess in the cMoroplasts <i mechanism for elaboraling cane sugar directly 



from, the carbon dioxide of the air. From the fact that cane sugar is the 



storage form in the sugar beet and mangold, the argument might be 



advanced, by those who regard dextrose as the primary product of 



synthesis, that the presence of cane sugar in the leaf of these plants is 



exceptional and due to there being here a special mechanism for its 



production. But we find that even in plants, like the potato (see 



D. 367), w'hich store starch as a reserve substance, cane sugar is the 



predominating sugar in the leaf; and even in the grape (Yitis vinifera), 



where de.xtrose is the storage form, we find that, when special precautions 



are taken in sampling to prevent the leaf enzymes from inverting the 



saccharose present, the latter sugar is the principal sugar of the leaf*. 



' Thi.s is quite contrary to Dcloano's stiitomcnl |1!)121 tlmt cane sugar is not present 

 in tlio vine leaf. Deleano's inahility to detect saccharose was probably due to insufficient 

 precautions being taken to prevent inversion. 



