168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
carbon, oxygen, and hydrogen, as in starch and sugar, compose the 
main part of plant substance. Some of the other chemical elements, 
to be sure, are necessary to healthy plants, though in very small 
proportions. The chemical formulae of some of these compounds 
are excessively complicated, and raise our admiration for plant 
chemistry. 
As plants must have light in order to grow, they strongly com- 
pete for it by stretching toward the sky. Where light is inadequate 
the stems lengthen. ‘This is called by botanists etiolation. Its effect 
is very marked in the comparative shapes of two pines, one growing 
alone on a clearing, the other in a thick wood. Another well-known 
effect of scarcity of light is to thin and broaden the leaves. This 
is taken advantage of by some tobacco growers, who by erecting 
semitransparent tents over their crops produce a higher grade of 
tobacco. 
It is at first sight quite surprising, but after all quite in harmony 
with the facts of etiolation, that plants grow tall faster in the night 
than in the day. Their maximum rate of growth is just after sunset, 
when it is apt to be over twice as rapid as in midday. 
There is also a curious expansion and contraction of plant stems 
in growing. The contraction seems to be caused by the rapid evapo- 
ration of water from the leaves during the daylight hours, and a 
resulting upward tension of the liquid in the conducting channels 
of the stem, which relieves the horizontal pressures to some extent. 
When we inquire which of the spectrum rays, and in what inten- 
sity, are required for plant growth and for seed formation, we find 
that a great gap in exact knowledge exists here. Most of the experi- 
ments hitherto made relate to plants of little or no commercial value, 
and lack exactness both as to the intensity and the quality of the 
rays used. It would be, indeed, diflicult and costly to employ the 
spectroscope to select rays for such experiments, because the use of 
a slit and numerous optical pieces so greatly reduces the intensity 
of the rays of every color. Most experimenters, therefore, have made 
shift to employ colored glasses to give certain rough separations of 
color. 
These experiments indicate that the blue and violet and ultra-violet 
rays are the most important for plant growth. Deep red rays also 
are very active to promote photosynthesis, but the green rays lying 
between these spectral regions seem to behave as darkness to the 
plant. It is greatly to be hoped that more exact measurements of 
wave length and intensity may soon be associated with studies of the 
growth and fruiting of the valuable food crops and the favorite 
flowers. It will be necessary to use very large and costly apparatus 
in such an investigation, because not more than 10 per cent of the 
intensity of sun rays may be expected to remain after the rays have 
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