F. F. Blackman 
4 
by spontaneous interactions. All this radiating flux represents the 
purely chemical background of possibilities out of which the cell 
forms its particular uni-directional stream-lines of metabolic flux. 
What we have to investigate in protoplasm is the mechanism that 
determines change in one direction rather than others, out of the 
chemically limited possibilities. 
To return to the fact that hexoses and pentoses occupy the first 
and second place in carbohydrate metabolism to the exclusion of the 
adjoining classes of tetroses and heptoses. Nef has shown that this 
is inherent in the structure of the different sugar molecules them¬ 
selves. When formaldehyde is condensed by lead hydrate in vitro it 
is found that the end-products are almost entirely derivatives of 
just these two classes of sugars. Lower sugar-classes have merely 
a transient existence as intermediate stages in the test-tube—as in 
the cell; and in neither does condensation go on to higher classes of 
sugars such as heptoses. 
We get real light on the cell and its possibilities and limitations 
from his general conclusion that just those individual hexoses which 
are abundant in plants, are exactly the ones whose derivatives tend 
to* accumulate in the test-tube, having a certain inherent degree of 
stability. Nefs picture leads us on further, to imagining traces of 
innumerable sugar derivatives and forerunners present in the cell, 
as transitional states. Research directed towards more complete 
identification of cell-activity with purely chemical activity in vitro , 
on these lines, is very difficult and hardly begun yet. Up to the 
present we have had to be content with identifying only those sub¬ 
stances which occur in bulk in the cell. 
Understanding now why hexoses are so abundant in carbo¬ 
hydrate economy, our attention may next be turned to pentoses. 
The drift of recent opinion is towards believing that pentoses 
really play a more important part in the cell than has hitherto been 
recognized. Pentoses, and their particular polysaccharides, pento¬ 
sans, are important constituents of the nucleus, of certain cell-walls, 
and of mucilage, but little general metabolic significance has been 
allowed them. 
Now, in a particular type of plant, pentoses and pentosans are 
abundant. Spoehr in his work on the carbohydrate economy of 
Cacti has emphasized their importance in relation to plants that 
exhibit the type of structure and physiology that is known as 
‘succulent/ Recently a remarkable pronouncement has been made 
on the real factor which determines the succulent habit, which habit 
