Physiological Studies in Plant Anatomy 225 
III. The Root Apex as an Absorptive Organ? 
In three recent papers Coupin( 3 , 4 , 5 ) has brought forward ex¬ 
perimental evidence in proof of his theory that the root apex is the 
chief if not the only organ for the absorption of water and solutes 
in the root. This view is in such direct opposition to the conclusions 
we have reached, both on theoretical and experimental grounds, 
that Coupin’s work will now be critically examined, other reasons 
suggested for his results and some experiments described which, in 
disproving his suggestions, lend further support to our evidence for 
the impermeability of the root meristem. 
In experiments with seedling peas, carried out in the dark, 
Coupin( 3 ) shows that whilst such a pea, suspended over water in a 
small conical flask, fails to grow and dies in a few days, another pea 
in the same flask with its root apex just immersed in the water 
grows as normally as could be expected. In a second paper ( 4 ) he 
compares the root development in various seedlings when grown 
(a) with the tip only in water, and ( b) with the whole root sub¬ 
merged. The plants were kept in a humid atmosphere in shade and 
at a constant temperature of 23 0 C. In the majority of seedlings 
experimented upon, growth proceeded much more vigorously when 
the root tips only were submerged, but in the case of white lupin 
and Mirabilis jalapa L., the roots showed approximately equal 
growth. Coupin also succeeded‘in growing the roots of maize and 
peas with the tips sealed into a narrow tube in which they were 
kept in moist air, while the rest of the root was completely immersed 
in water. Under these conditions very little growth took place until 
the appearance of secondary roots, when Coupin assumed that the 
root was enabled once again to absorb water through the apical 
meristem. From these experiments Coupin concludes that roots 
absorb water exclusively by the apex, the root hairs are assumed 
to be a protection against too rapid evaporation and to provide the 
necessary support for the root apex when penetrating the soil. 
The following criticisms seem relevant. In the first case, upon 
the submergence of the first two or three millimetres of the tip in 
water it seems certain that water will be drawn up the surface of 
the root and through the cellulose walls of the cortical region by 
surface tension, so that the normal absorptive region of the root 
obtains a plentiful supply. 
The later experiments are much more striking but it is suggested 
that they indicate rather the retarding effect of complete immersion 
