416 
PHYSIOLOGY. 
BOOK II. 
are due to the tendency of the less coloured part to avoid the 
light, which is manifested by bearing down the flower so as to 
approach the seat of obscurity as nearly as possible : all such 
flowers being always nodding. This tendency is aided by 
the weakness of the peduncle, which seems to have been 
specially provided for enabling such flowers to retire from the 
light. In papilionaceous plants, the inside of the vexillum, 
which is most deeply coloured, always turns itself towards the 
light; and the alae twist themselves half round, to effect the 
same object. The ovaries often take a different direction after 
the fall of the corolla from what they had before. Thus, during 
flowering, the ovary of Digitalis purpurea is nodding like 
the flower, the direction of which it is compelled to follow : 
immediately after the fall of the corolla, it turns upwards to- 
wards the light, to which it is attracted by its green colour. 
A contrary phenomenon is presented by the ovary of Con- 
volvulus arvensis. The flower is turned towards the sky : as 
soon as it has fallen, the ovary takes a direction towards 
the earth, bending down the peduncle^ This cannot be due 
to the weight of the ovary, which is much lighter than its 
peduncle, but must depend upon its disposition to avoid the 
light, on account of its pallid hue, which is nearly the same 
as that of the root. In Calystegia sepium, on the contrary, 
in which the ovary is equally pale, its erect position is 
maintained, and the influence of decoloration counteracted by 
the greater affinity to the light of two large green bracts in 
which it is enveloped. 
From the following and some other experiments, Dutrochet 
infers that the direction of leaves to the light is not mecha- 
nically caused by the operation of an external agent, but is 
due to a spontaneous motion, put in action by the influence 
of external agency. He took a leaf and cut off its petiole, the 
place of which was supplied by a hair, hooked by one end 
upon the leaf, and having a piece of lead attached to its oppo- 
site extremity. They were plunged in a vessel of water : the 
weight of the lead carried the leaf to the bottom of the 
water, where it stood erect in consequence of its lightness 
inducing it to attempt to ascend. Being exposed in a win- 
dow, so that the under surface was turned to the light, no 
f 
