Physiological Studies in Plant Anatomy 59 
a year ago. These will now be stated in terms of the problem of the 
causal anatomy of the Angiosperm, although in later papers data are 
utilised from any group of vascular plants. 
In the Angiosperm seedling certain internal factors are already 
present which mould subsequent development; these have been 
determined, either genetically, or during the maturation of the 
embryo within the developing seed. Thus, whether the seed is endo- 
spermic or non-endospermic, there is always an equatorial region of 
food supply, directed through an intermediary axis towards two 
polar, opposed meristems. These apical meristems already show 
different tropistic reactions and developmental characteristics which 
serve to distinguish stem and root. A later paper (IV) shows that in 
certain growing points more closely studied, the physiological differ¬ 
ences between the apical meristems have a suggestive relation to the 
morphological divergence between stem and root. These are con¬ 
nected with the effect of the structure of the meristem upon the 
conditions under which food is supplied to support further growth. 
As the problem of this supply of food to merismatic tissues arises 
over and over again in the course of the work, attention is drawn to 
certain relevant considerations of a general nature. Reasons are 
given later for regarding a merismatic cell as remaining non-vacuo- 
lated only so long as it is actively engaged in the synthesis of proto¬ 
plasm, although connected with the rest of the plant by a tract of 
vacuolated cells. Changes in the permeability of this cell allowing 
entry of water, mean vacuolation, a thickening of the wall and, most 
important of all, the loss of merismatic activity. The actively grow¬ 
ing merismatic cell must therefore be able to obtain the necessary 
nutriment from the solutes in the sap bathing its protoplast, but at 
the same time it must prevent the entry of water in which these 
solutes are dissolved; a problem which is considered in detail 
later (IV). 
For the moment the problem is the source of the necessary organic 
solutes. The only channel for their passage is along the walls of the 
vacuolated cells bordering the meristem, there being usually no inter¬ 
cellular spaces immediately abutting upon a meristem. Water will 
always be moving along the walls of these vacuolated cells in the 
adjustment of the constantly changing equilibrium between proto¬ 
plast and turgid wall, but if organic solutes are withdrawn from this 
water by the adjacent meristem there is little likelihood of a renewal 
of the supply by outward diffusion from these recently vacuolated 
protoplasts. The only possible source, therefore, of a constant and 
