580 Burns . — Heterophylly in Proserpinaca palustris , L . 
Two of the questions which arise in a study of variations in plant- 
organs are, in how far are they determined in form and structure by 
external stimuli ? and, do these show a direct adaptation to the environ- 
ment ? This paper deals largely with the first question. 
Most of the literature upon this subject deals primarily with the effect 
of external stimuli on the anatomical structure of the organs of amphibious 
plants. This side of the question is omitted in the present paper, although 
it is of great importance, because McCallum (3) promises a detailed account 
of the results of his experiments in the near future. For this reason the 
results of the work along this line will be briefly given. 
Mer (4) refers the differences in structure to weakened illumination 
and poor nutrition. Constantin (5) finds that the aquatic form is due to 
poor vegetative conditions. Schenck (6) agrees on the whole with Mer. 
On the other side of the question must be noted first the work of 
Goebel (7). In several cases he has shown that the form of the leaf 
is determined by external stimuli. This is true of Sagittaria , Campanula 
rotundifolia , and the Cacti. In these plants it depends upon the intensity 
of the light. In other cases, as Ranunculus midtifidus , the form of leaf is 
dependent upon the water-environment to the extent, at least, that the 
leaves formed in water are more finely divided than those formed in air. 
However, in some cases, notably in the case of Lininophylla heterophylla, he 
could prove no direct relationship between the form of leaf and external 
stimuli. He thinks, however, that a direct causal relationship may have 
formerly existed even though it cannot now be proven (Org. p. 546). 
McCallum (3) attempted to analyze more carefully the morphogenic 
factors working upon water-plants. From experimental work on Proser^ 
pinaca palustris , he concludes that the stimulus to the development of the 
water-form of leaf in this plant is not involved in the light-relations, in the 
nutritive conditions, temperature, the gaseous content of the water, nor 
contact-stimulus, but is due to the ‘ checking of transpiration and consequent 
increased amount of water in the protoplasm.’ He continues, f When the 
protoplasm of the primordial cells is in that condition of dilution which 
accompanies the absorption of a large amount of water, the nature of the 
growth and the orientation of the cell-division is such as to produce 
the water-form, while those physical and chemical conditions resulting from 
a partial withdrawal of water by evaporation (i.e. an increased density of 
protoplasm) result in that sort of cell-behaviour which produces the air-form 
of leaf.’ 
One other paper is that of Familler (8) on Campaimla rotimdifolia. 
He found that cuttings from stems of this plant, which were producing 
linear leaves, returned to the formation of round leaves. As we have seen, 
Goebel got the same reaction on this plant by decreasing the amount 
of light. Thus not only light, but a disturbance of vegetative activity — such 
