NUTRITION 375 
duced them. In addition to the staple crops just named, whose aggregate value 
in 1909 was about $3,000,000,000, other farm crops add nearly as much more, 
being estimated at $2,700,000,000. Such are the values that plants annually pro- 
duce in this country, chiefly from the air and water, by photosynthesis. 
Process. The process of photosynthesis is not certainly known; 
but all the evidence points strongly in one direction; so that the hypoth- 
esis of von Baeyer may be considered as highly probable. It appears 
that the carbonic acid (CO 2 + H 2 O ^OH-COOH) is by some means 
reduced, perhaps first to formic acid (H-COOH), and later to the sim- 
plest carbohydrate, formaldehyde (H-COK). In the course of this 
reduction a molecule of oxygen, O 2 , is set free and appears as a by- 
product. 
The reduction of H 2 COs to formaldehyde has lately been accomplished artifi- 
cially, though much less efficiently than in plants. A thin layer of chlorophyll on 
gelatin or floating on the surface of water (to which has been added an enzyme that 
will break up hydrogen peroxid, H 2 O 2 , into water and oxygen), when supplied with 
CO 2 in light permits the accumulation of formaldehyde and oxygen to a measurable 
extent in the apparatus. The formaldehyde molecule so quickly combines with 
others of its kind that it has been difficult to prove its formation in leaves. Free, 
it is a powerful poison, even in dilute solution (i : 20,000); but its prompt conden- 
sation into some hexose sugar prevents accumulation to a harmful extent. The 
'H 
details are probably as follows : six molecules of formaldehyde, H C^ , unite 
into a chain. This union engages two of the four bonds of each C atom, except at 
the ends, where only one is concerned. This consequently either releases one of 
the two O bonds or leaves one H atom free, or does both. The free H immediately 
joins its neighboring half -free O, and together they form OH, bound to C by only 
one bond. At one end no H is freed ; but the half-freed O takes up H and the group 
becomes CH 2 OH, characteristic of an alcohol. At the other end, the loss of one 
/H 
H leaves the aldehyde group C ^ as in formaldehyde. In glucose a further 
^o 
transposition occurs in group 4, H and OH exchanging places. 
H H H OH H 
H C C C C C C^ = rf-glucose (p. 359). 
I I I I I ^O 
OH OH OH H OH 
Glucose and starch. Glucose probably represents the first, stable 
carbohydrate formed in most plants; yet there is some variation in this 
respect in different plants, and there is evidence that in some cases cane 
sugar, saccharose, is the chief product. It is quite possible, moreover. 
