Physiological Studies in Plant Anatomy 219 
The relative differences in permeability in these two meristems 
must be explained by the composition of the intervening walls, the 
protoplasts in both cases must remain impermeable to water if the 
arguments advanced in the previous section are sound. The results 
of a comparison of the walls of these two meristematic tissues are 
given in the following section. 
(3) Composition of the Wall at the Apical Meristem of 
Stem and Root 
The most striking difference between the two apices is their 
behaviour towards the usual reagents for cellulose. The walls of the 
meristematic cells of the stem apex right up to the layer below the 
cuticle, treated with iodine in potassium iodide and strong sulphuric 
acid, give a strong cellulose reaction. On the other hand, when 
sections of a root apex are similarly treated, the walls of the meri¬ 
stem without exception show no trace of cellulose. The cell walls 
of the root cap usually give a strong cellulose reaction, as do also 
the walls of periblem and plerome at varying distances behind the 
growing point. The plerome remains free from cellulose for a greater 
distance back than the periblem. This microchemical behaviour of 
the membrane at the root apex has been previously reported by 
Solla ( 26 ), quoted by Gardiner ( 7 ). 
The walls frequently are described as of “young cellulose/’ thus 
Molisch ( 18 ) cites a staining reaction with congo red introduced by 
Zimmermann as a test for young cellulose in root tips. This reaction 
with congo red, however, has marked resemblances to the staining 
behaviour of congo red with proteins such as egg albumin, and has 
only become known as a reaction for “young cellulose” because it 
has been assumed that the apical meristem must contain such a 
substance. Heidenhain, quoted by Mann ( 17 ) (loc. cii. p. 455), de¬ 
scribes congo red, which is the sodium salt of a sulphonic acid, as 
forming with albumin a congo red albuminate which is transformed 
into a darker red albumin congo red sulphonate on addition of 10 
per cent, acetic acid. The free blue insoluble congo red acid is only 
liberated from this compound with albumin on addition of a strong 
mineral acid. When sections of the root apex of Salix fragilis (L.), 
from roots grown in water, were left for twenty-four hours in congo 
red, the walls of the meristem were stained a deep red which only 
changed to a darker shade on addition of 10 per cent, acetic acid 
whilst the stain in the cellulose walls in the older regions of the roots 
promptly went blue. It was necessary to add a mineral acid before 
15—2 
