326 HOW CROPS GROW. 
Oxygen Gas needful to AssimiJation.—Traube has made 
some experiments, which seem to prove conclusively that 
the process of assimilation requires free oxygen to surround 
and to be absorbed by the growing parts of the germ. 
This observer found that newly-sprouted pea-seedlings 
continued to develope in a normal manner when the cot- 
yledons, radicles, and lower part of the stem, were with- 
drawn from the influence of oxygen by coating with var- 
nish or oil, On the other hand, when the tip of the 
plumule, for the length of about an inch, was coated with 
oil thickened with chalk, or when by any means this part 
of the plant was withdrawn from contact with free oxygen, 
the seedling ceased to grow, withered, and shortly perish- 
ed. Traube observed the elongation of the stem by the 
following expedient. 
A young pea-plant was fastened by the cotyledons to a 
rod, and the stem and rod were both graduated by deli- 
cate cross-lines, laid on at équal intervals, by means of a 
brush dipped in a mixture of oil and indigo. The growth 
of the stem was now manifest by the widening of the 
spaces between the lines; and by comparison with those 
on the rod, Traube remarked that no growth took place 
at a distance of more than 10-12 lines from the base of 
the terminal bud. : 
Here, then, is a coincidence which appears to demonstrate 
that free oxygen must have access to a growing part. 
The fact is further shown by varnishing one side of the 
stem of a young pea. The varnished side ceases to extend, 
the uncoated portion continues enlarging, which results in, 
and is shown by, a curvature of the stem. 
Traube further indicates in what manner the elabora- 
tion of cellulose from sugar may require the codperation 
of oxygen and evolution of carbonic acid, as expressed by 
the subjoined equation. 
Glucose. Oxygen. Carbonic Acid. Water. Cellulose. 
2 (Cig Hog O12) + 240 = 12 (CO2) + 14 (H20) + Cro Hoo Oro. 
