42 
J. H. Priestley and J. Ewing 
It is clear without further discussion that the changed behaviour of 
the developing membranes of the etiolated tissue is very suggestive 
in relation to etiolation phenomena in widely removed groups of 
plants. 
Summary 
1. The plant may be regarded as a dynamic equilibrium between 
internal inherited factors and the external environment during de¬ 
velopment. 
2. The root and stem normally develop in very different environ¬ 
ments: if these are reversed experimentally, an opinion may be 
formed regarding the extent to which the normal structure is deter¬ 
mined by internal factors. 
3. If the root is grown in light and air relatively little structural 
modification can be seen in it. 
4. But if the stem is grown in darkness very great structural 
modifications are produced: these are known as the phenomena of 
etiolation. 
5. Various types of etiolation are exhibited by plants of different 
growth habit, but in this paper the discussion is restricted to that 
very common type exhibited by the broad bean and potato. 
6. It is shown that in this case an explanation is possible in terms 
of causal anatomy on the assumption that when the stem apex grows 
under conditions more normal to the root apex, its development pro¬ 
ceeds in a manner more characteristic of the root. 
7. Thus the walls between the protoplasts of the apical meristem 
remain relatively impermeable to the nutrient sap. The “plumular 
hook” therefore persists and meristematic tissue active in growth is 
only found below it. 
8. The rudiments of the lateral leaves and axillary branches 
therefore fail to develop further; these etiolated leaf rudiments are 
rich in protein, but lack water, carbohydrates and inorganic salts. 
9. Another consequence of the changed meristematic develop¬ 
ment is the production of a functional primary endodermis in the 
stem. To the presence of this endodermis may in part be attributed 
the reduced cortical development and the lack of angular or winged 
contours of the etiolated stems in cross section. 
10. Adventitious roots develop freely in the etiolated plant as 
the result of the distribution of the nutrient sap determined by the 
presence of the endodermis; there is good reason to think that a 
pericyclic periderm, as opposed to a normal periderm, may often 
arise as the result of the same factor. 
