Physiological Studies in Plant Anatomy 41 
showed most vigorous growth, and then in order, those in Shive’s 
optimal solution, those in Shive’s supra-optimal, while those grown in 
the "pea extract showed a very dwarfed and unhealthy condition, as 
if the extract had some strongly toxic action on them. It was thought 
that the high osmotic concentration of the extract and the lack of 
oxygen might have had some inhibitory effect on the growth of the 
roots, and the experiment was accordingly repeated, the original 
extract being this time diluted 1:99 with tap water and oxygen 
bubbled through the liquid. The controls were set up the same as 
previously. In this case the roots growing in the extract showed 
slightly more growth, but the shoots attained a length uniform with 
those of the controls. Until further details of his experiments are 
given by Coupin.we can only conclude that the shortened growth of 
his etiolated stems was merely an expression of the toxic effect on 
growth produced by the solution applied to the roots. In any case 
Coupin’s suggestion that the excretion from the chloroplasts in the 
light thus supplied to the etiolated plant prevented it assuming its 
normal etiolated appearance would be difficult to reconcile with the 
frequently observed experimental fact that etiolated portions of 
plants, organically connected with another part of the same plant 
growing in the light, continue to show all the normal phenomena of 
etiolation. 
In conclusion it is submitted that the account given above of the 
causal sequence of events leading to the etiolation phenomena charac¬ 
teristic of plants of the broad bean type, gives a consistent picture of 
the structural and developmental features brought into play as the 
result of growth in darkness. In the light of the developmental se¬ 
quence at the stem apex and the consequent structural alteration 
behind that apex, all the characteristic phenomena of etiolation of 
this type may find their explanation. Other growth habits will lead 
to other types of response to etiolation, and in particular the etiolation 
habit of the Monocotyledon with its basal leaf growth is reserved for 
later consideration. 
Similarly special cases, such as the lack of response to etiolation 
in Humulus (Sachs (22)), will also be dealt with on another occasion. 
These allied problems have already been under investigation, and it 
is hoped to show later that they admit of solution, from the stand¬ 
point of causal anatomy, without assumptions that conflict with the 
general hypothesis presented in this paper, which itself is the natural 
consequence of the direction from which the general phenomena of 
plant anatomy have been under investigation in this series of papers. 
