36 EMBRYOGENESIS IN PLANTS 



metabolites. Thus, in the present writer's view, the theory would 

 appear to afford an explanation of the inception of the symmetrical, 

 radiate histological pattern that appears adjacent to the embryonic 

 region of the root apex. Not all kinds of pattern, however, are referable 

 to the development of stationary waves— the major feature of Turing's 

 theory as thus far developed— but all may eventually be related to some 

 kind of diffusion-reaction system. The inception of polarity, i.e. of 

 axial development, in an embryo is probably due to a particular distri- 

 bution of metabolites in an initially homogeneous system; this could 

 be regarded as a very simple case of a stationary wave. The following 

 may be tentatively indicated as examples of pattern in plants which 

 may perhaps be explained, in whole or in part, as the theory is 

 more fully developed and explored: phyllotactic systems; whorled 

 branching in algae; the distributing of procambial strands in shoots; 

 the radiate pattern in root steles and in lycopod shoots. Turing has 

 indicated how the dappled pattern in the skins of animals and 

 gastrulation in the developing animal embyro can be explained by his 

 theory. 



In the general system of ideas incorporated in the theory there are 

 many points of interest to the student of morphogenesis. Thus, with 

 regard to the breakdown of symmetry and homogeneity, attention is 

 directed to the importance of small random changes in the distribution 

 of morphogenetic substances, i.e. irregularities and statistical fluctua- 

 tions in the numbers of molecules taking part in the various reactions. 

 The determination of polarity in the fertilised ovum of Fucus, for 

 example, may be due to random changes, or to factors in the environ- 

 ment. In the enclosed embryos of land plants, in which polarity is 

 determined soon after fertihsation, if not before, quite small gradient 

 effects proceeding from the gametophyte tissue could be the means of 

 initiating the breakdown of homogeneity and the establishment of 

 polarity. Some deviations from homogeneity in a reaction system may 

 be of great importance in the process of differentiation; for the system 

 may reach a state of instability in which the irregularities, or certain 

 components of them, tend to grow. If this happens, a new and stable 

 equilibrium is usually reached, and this may show a considerable 

 departure from the original distribution of metabolites. Thus, in 

 contiguous cells which are initially metabolically identical, a drift from 

 equilibrium may take place in opposite directions as a result of statistical 

 fluctuations in the components of the reaction system, or of small 

 changes induced by neighbouring cells. Changes of this kind could, for 

 instance, account for the very different developments in two adjacent, 

 equivalent embryonic ceHs — a histological phenomenon of much 

 interest to the botanist. 



