GROWTH AND MOVEMENT 
437 
Light and form. The form of the aerial parts of mosl plants is pro- 
foundly influenced by light, directly or indirectly. This is shown by the 
striking changes that ensue (etiolation) when they are 
grown in darkness. Without starvation this is pos- 
sible only with plants that have already stored a 
sufficient amount of surplus food. One who has 
observed the long pallid shoots of a potato which 
has sprouted in the dark will have seen the general 
effects. The stems tend to elongate much more than 
usual, though they are not necessarily more slender; 
the branching is at a different angle; and the leaves 
remain small and imperfectly developed. (The pallor 
from lack of chlorophyll and the presence of carotin 
are features already mentioned.) On the whole, 
elongation is likely to be accentuated, breadth is likely 
to be repressed (fig. 676). Though these are the 
common results of the lack of light during develop- 
ment, they are by no means universal. Thus, there 
are plants whose stems do not elongate, and others 
whose leaves are not reduced. But if not these, 
other characteristics may be altered; e.g. reduction 
of the mechanical elements of the tissues is one of 
the less obvious effects. Scarcely a plant escapes but 
those that pass all their lives in darkness, and only 
those parts that are buried in the soil are exempt from 
the formative influence of light. 
Dorsiventrality. In plant organs not grown in 
darkness, but of which one side is better illumi- 
nated than the other, light effects can be observed. 
One effect is the development of a distinctly different 
structure in the better lighted surface as compared 
with the shaded one, and since these are naturally the 
upper and under surfaces, an organ showing such 
differences is termed dorsiventral. 1 Thus the pali- 
sade portion of the mesophyll of leaves owes its exist- 
ence chiefly to light. 2 Dorsiventrality in the liverworts is likewise due 
mainly to light. None shows this better than the common Marchantia. 
1 Dorsiventral organs may owe the difference of their faces to other formative stimuli, 
e.g. to gravity. See footnote, p. 421. 
FIG. 676. Plant 
of Phaseolus grown 
in darkness. After 
MACDOUGAL. 
