[December, 
708 Sanitary Agency of Light. 
effect is produced by the more refrangible rays, and may be 
classed with the chemical phenomena of photography. 1 he 
sanitary aspedl of the subject is only a particular applica- 
tion of a far-reaching law, including probably all animal and 
vegetable life ; and the results of it, so far as botany is con- 
cerned, have hardly received the recognition they deserve. 
There is a simple experiment in illustration which anyone 
may make for himself, and which is highly suggestive. Let 
a bean be put in a little water at the bottom of a tin 
canister with the cover on (a mustard tin will do well), and 
kept a day or two in a warm place ; on removing the lid, it 
will be found that the plumule of the seed has shot up 
into a long, straight, succulent stem. Now make a hole in 
the tin in such a position that light may fall through it 
upon the stem about one third of its length from the top. 
It will be found that the shoot gradually bends oyer 
towards the side where the hole is, the curve beginning 
where the light falls ; if a longitudinal section of the stem 
be made, and examined with a low power under the micro- 
scope, the cause of the curvature will be clearly seen. On 
the concave side the cells are shortened, thickened, and 
generally huddled together, so that the top has been drawn 
down towards that side, the effedt being evidently due to 
the check imposed upon the growth of their protoplasmic 
contents by the adtion of the light. Whether the thicken- 
ing of the cell walls implies an increased growth of cellulose 
(or woody matter) is uncertain, but it is more probable that 
it is due to the spreading of the same amount of material 
over a smaller surface. It is to be specially noticed that 
the bending of the stem, and the thickening of the cells, 
precedes the appearance of a green colour, which, however, 
soon follows if the exposure is prolonged. 
This little experiment throws a flood of light upon a large 
class of obscure phenomena connedted with the growth and 
habits of plants; it shows why “ etiolated ” specimens ( i.e 
plants grown in the entire or partial absence ol light) tend 
to become spiry, and to throw out lengthened shoots ; it 
will account, too, for many of the observed fadts of “ helio- 
trophy” — the turning of plants towards the sun or light — • 
which have previously all been regarded as purely vital 
phenomena. We seem, also, to get a glimpse at the mean- 
ing of the appropriately named “ pileus ” or cap of. a 
numerous class of fungi represented by the common mush- 
room, under which the delicate highly-vitalised spores are 
stowed snugly away out of reach of the light. We find out, 
too, why the delicate CEthalium, or “ flowers of tan ” — a 
