ETIOLATION AS A PHENOMENON OF ADAPTATION. 351 
also abnormally narrow* — in other words, the energy of growth 
is thrown into the direction that leads to escape. As soon, how- 
ever, as they are illuminated, they grow in width ; that is to say, 
when elongation is no longer necessary, the leaf can afford to 
increase in width, and thus to gain a greater assimilating area. 
Similar facts have been observed in the radical leaves of some 
Dicotyledons, where, in the same way, abnormal elongation is 
the only hope of escape into daylight. In some leaves again the 
same principle is followed with a difference : the lamina remains 
dwarfed while the petiole is greatly elongated. Similar facts 
may be gathered from Vochting's t curious observations on Cacti, 
which in continuous darkness fail to develop the wing-like 
expansion of their stems, so that, like the narrow leaves of 
etiolated Monocotyledons, they do not increase in assimilating 
area without the stimulus of illumination. The nutrition theory 
cannot explain these facts, nor is it any help to refer them to the 
*' tonic influence " of light — a restatement of fact which is some- 
times put forward as an explanation. 
A few outlying or exceptional cases may be noted. In 1863 
Sachs pointed out that climbing plants, e.g. the Hop, are excep- 
tional in the matter of etiolation. He stated that a Hop-stem 
developed in complete darkness does not exhibit the exaggerated 
elongation so characteristic in one type of etiolation. 
In other words, the internodes of the darkened plants were 
practically of the same length as those of the normal specimens 
cultivated in the light. I am inclined to believe that this case 
is explicable when the biological meaning of the climbing habit 
in plants is taken into account. A climbing plant is one whose 
whole organisation is adapted to reaching light at a small ex- 
penditure of solid matter, so that its length of stem in proportion 
to its weight is enormous in comparison with the same relation 
in plants which support their own weight. It comes to this : 
the normal, light-grown Hop is already exaggerating internodal 
growth to the utmost in its search for light — so that etiolation 
cannot produce its usual effect. The Hop is not a solitary 
instance. Sachs remarks that the vernal shoots of the Sweet 
Potato, developed normally in daylight, have the habit of 
* They also remain folded up, which, as Godlewski [loc. cit. p. 486) points 
out, would favour their emergence from the soil, 
f Prmgsheim's Jahrbiicher, xxyi. 1894:. 
