SEPTEMBER 8, 1899. | 
exact measurements of the amounts of en- 
ergy involved—a thing not yet accom- 
plished—before we can be said really to 
know whence plants derive their energy 
and what heat means for them. 
Even the case of the green plants is not 
at all clear. That they construct their own 
food in great. measure is certainly true. 
That they do this by using absorbed radiant 
energy of the quality which gives us the 
sensation light is well known. But it is by 
no means clear, in terms of chemistry and 
physics, how this is done, or even what 
measure of the absorbed energy is utilized. 
Measurement, indeed, is difficult, yet quan- 
titative results are necessary before we can 
be satisfied that we know what is happen- 
ing when the leaf makes food. 
Finally it may be said that little is yet 
known of the energy relations in the proc- 
esses of growth. Here, since we must 
deal wholly with internal release and 
utilization of energy, investigation will be 
most difficult and uncertain. 
Stereochemistry.—The decade that is pass- 
ing has witnessed the very great extension 
of chemical knowledge in the direction of 
the constitution of the molecules of carbon 
compounds. Stereochemistry touches plant 
life most obviously in its relation to the 
carbohydrates, which are constructed by the 
green plants, and digested and utilized by 
all. The phenomenal work of Fischer on 
the sugars, supplemented as it has been by 
that of Tollens, Kiliani, Lobry de Bruyn 
and others on asymmetric carbon atoms, 
has put us in possession of facts which 
throw a flood of light upon nutrition and 
are destined when more completely ex- 
ploited and fully applied, to elucidate many 
difficulties in our present thinking about 
the feeding of plants. 
We have learned for example that a car- 
bon compound, to be a valuable food, must 
not only contain C, H and O, but that these 
must be combined ina particular fashion. 
SCIENCE. 
321 
The aldehyde group CHO, the ketone 
group CO, and the radical CH,OH are char- 
acteristic of good foods. The simpler sug- 
ars such as glycerose and arabinose; the 
hexoses, glucose or grape sugar, fructose, or 
fruit sugar, mannose and galactose; the 
polysaccharides, sucrose or cane sugar, lac- 
tose or milk sugar, and maltose or malt 
sugar are all substances which have been 
proved useful as plant foods, and all con- 
tain one or more of these groups. 
Up to a certain limit, the presence of a 
particular molecular group increases the 
food value. What does this phrase ‘ food 
value’ mean? Does food value depend’ 
solely on availability of energy, 7. e., the 
ease with which it can be released? Or has 
the form in which energy is set free some- 
thing to do with its availability and the 
consequent food value? Or does the con- 
stitution of the molecules before and after 
decomposition affect food value? If so, is 
it because the constitution of the molecules 
is related to the form in which energy is re- 
leased or because itis related to the ease 
with which energy is released ? 
When the complex carbohydrates like 
starch and inulin are to be utilized, they 
break down through a series of dextrins 
and levulins respectively, finally becoming 
simplified to hexose sugars. Why is this 
necessary ? And how are we to interpret 
these decompositions? Are they part of 
the energy-release? It can hardly be 
doubted that the constitution of the mole- 
cules of starch and innlin, composed as they 
are of units of glucose and fructose, deter- 
mines the permanence while in the storage 
form, and that separation into their con- 
stituent units in digestion makes possible 
the assimilation of the sugars as food. It 
is plain, therefore, that a precise knowledge 
of the constitution of starch and inulin is a 
desideratum. We must look forward also 
to further extension of stereochemical 
knowledge of the almost infinite variety of 
