Physiological Studies in Plant Anatomy 37 
growth habit, and Wiesner(26), when making a comparison between 
growth in darkness and growth in a saturated atmosphere, distin¬ 
guished still further types of reaction to continued darkness. This 
varying behaviour shown by different plants under the same con¬ 
ditions, increases the complexity of the problem, and is perhaps 
responsible for the fact that some investigators have taken refuge in 
purely teleological explanations of etiolation, e.g. Francis Darwin (4) 
and Godlewski( 6 ). From the present point of view teleological argu¬ 
ments are inadmissible, and an explanation in terms of causal 
anatomy is advanced for one type of growth reaction distinguished by 
Sachs and Wiesner, viz. that very common type which is exemplified 
by the broad bean and potato. 
The Etiolated Leaf 
Sachs distinguished between leaves which emerge from the shelter 
of the bud at an early date and continue their growth fully exposed 
to light when distributed along an extended stem. Continued dark 
ness completely prevents the further development of a rudimentary 
leaf of this type. On the other hand, leaves that grow by the activity 
of a basal region which remains sheltered at the base of a shortened 
internode continue to grow in darkness. 
The broad bean is representative of the type in which the leaf 
initial is carried into the light by the elongating internode, and in this 
type most observers will agree with Sachs’ earlier statement that 
this type of leaf completely fails to develop if maintained in continued 
darkness. Confusion has arisen in certain cases, and Sachs himself 
reported later that the leaf of Cucurbita Pepo , previously described 
as undeveloped, reached almost its full size in the dark. Frank (15) 
(loc. cit. p. 395 ) found the etiolated leaves of this plant of the normal 
undeveloped type, but Jost(7) and Teodoresco(24) have since reported 
that the leaves reach their normal size in the dark if provision is 
made for their nutrition. These conflicting observations appear to be 
due to neglect to distinguish between the growth of leaves upon a 
bud which has never been exposed to the light and upon a bud that 
has been for a short period at any rate in normal daylight before 
being maintained in continued darkness. 
Bataline(i) thought that failure of leaf rudiments to develop under 
etiolation conditions arose from the inability of the meristematic 
cells to divide, but Prantl(i4) [loc. cit. p. 384 ) has shown by actual 
countings, that there is a considerable if slow increase in the number 
of meristematic cells, but that these cells never undergo the increase 
