186 W. H. Pearsall and J. H. Priestley 
case the first sign of meristematic activity—as seen under the 
microscope—is that aggregates of protoplasm appear in the originally 
vacuolated cells, and from these aggregates new non-vacuolated or 
only slightly vacuolated cells are cut off. 
Cylinders of meristem such as the vascular cambium and the 
cork phellogen are characteristic features of Dicotyledon anatomy, 
and in order to understand the causal machinery concerned in their 
production it would seem essential to know how the original dense 
aggregation of protoplasm in the vacuolated cell is brought about, 
and to determine the conditions necessary for its maintenance. The 
phenomenon recalls the precipitation of protein from a sol, or the 
contraction of a protein gel with loss of water. These are processes 
upon which a considerable amount of light has been thrown by 
recent investigations in physical chemistry. The object of this note 
is to show that the appearance of these meristems seems to be 
correlated with a gradient of hydrogen-ion concentration in the 
tissues in which they arise and that in the light of recent physico¬ 
chemical investigation this fact may have very great significance. 
The Gradient of Hydrogen-ion Concentration across a 
Cambial Zone 
Many writers have observed (see Atkins (i)) that it is now possible 
to obtain a rough idea of the reaction (hydrogen-ion concentration) 
of plant tissues by employing the modern range of indicators (5) in 
contact with sections of fresh material. When the tissues near an 
active cork cambium are examined in this way, the newly-formed 
cork cells are found to be extremely acid in reaction, approximately 
pH 3-0, this being due apparently to the fatty acids liberated from 
the rapidly differentiating cork cells (Priestley and Woffenden (9)). 
On the other hand, within the phellogen, the cortical layers of paren¬ 
chyma have normally a reaction between pH 5-5 and 6-5. Hence 
the cambial layer lies across a fairly steep gradient of hydrogen-ion 
concentration. 
In the case of the cambium of normal vascular tissues the con¬ 
dition is somewhat different. Atkins has recently stated (1) and we 
have frequently observed, that the reaction of the xylem is relatively 
acid (pH 4-3 to 5-0), whilst, as Sachs pointed out many years ago (11), 
the phloem is relatively the most alkaline tissue in the plant and 
may frequently be alkaline to litmus (pH 7-8 or more). It is therefore 
a striking fact, and one worthy of much further investigation, that 
each of the two cambial meristems of the normal Dicotyledon lies 
