452 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
from the integral substance of the cell itself as soon as all 
reserve carbohydrate, which is never very abuudant in actively 
vegetating Elodea plants, is consumed. The same causes which 
accelerate respiration (fairly high temperature, abundant supply 
of oxygen, &c.) also hasten the production of the degenerative 
changes. In sugar solution the change cannot possibly be a 
phenomenon of starvation, though it may be due to mal- 
nutrition of the cell. Very possibly it may be a pathological 
condition induced by the abundant supply of sugar, the now 
useless chlorophyll bodies wasting away and degenerating, espe- 
cially if subject to the changes and decompositions caused by 
exposure to the action of light. 
The Age of the Leaf Cell or Chlorophyll Grain at which 
Assimilation commences. 
The power of assimilation of a chlorophyll grain when once 
fully formed in an adult cell remains active under normil 
conditions during the life of the cell, and only begins to 
diminish when the vitality of the cell is on the wane. The 
division of a chlorophyll grain, as has already been seen, to 
form two new grains does not involve any cessation or diminution 
of the power of assimilation. When, however, the chlorophyll 
bodies are being formed in a developing tissue, ab initio, by 
protoplasmic differentiation, they are at first colourless and 
necessarily without any power of assimilation. To determine 
more exactly the age of the leaf or leaf-cell at which the chloro- 
phyll grains become capable of assimilating, a series of observa- 
tious were carried out in spring upon the young developing leaves 
of a variety of plants. When the leaf is thin and the inter- 
cellular spaces few and small, a portion of the lamina free from 
veins may be examined; but in all other cases sections are used. 
Whenever no power of assimilation can be detected, the obser- 
vations are repeated. The measurements given are in terms of 
the largest adult leaf; thus L. 4, B. 4, P. Y means that the 
length and breadth of the lamina and the length of the petiole of 
the young leaf bear to those of the adult leaf these relations. In 
the first column the measurements are given of the largest leaf 
in which no power of evolving oxygen can be detected, the sub- 
sequent columns tracing assimilation from its first appearance 
until itis fully active. Where not otherwise stated, the measure- 
ments and determinations at any given time are taken from the 
