26 SCIENCE PROGRESS 



In South Africa the xerophytic environment is widespread, 

 and the mesophytic one local, the latter representing later 

 stages in the succession. It is suggested that the contact 

 between widespread species and more mesophytic, or sometimes 

 more xerophytic, conditions has been the stimulus for the 

 mutation of these disconnected types. A number of species 

 are cited in illustration. 



PLANT PHYSIOLOGY. By R. C. Knight, D.Sc. Imperial College 

 of Science and Technology (Plant Physiology Committee). 



The Root System. — Probably owing to the practical difficulties 

 of experimentation, the study of the physiology of the root 

 system had not in the past progressed at the same rate as the 

 study of stems and leaves. Of late, however, the literature 

 contains a relatively large number of reports of work, both 

 observational and experimental, on the relation of roots to 

 environment and on the individuality of the roots themselves. 

 The methods which have been used for investigation of root 

 problems are unfortunately few, consisting mainly of the 

 variation of the water, oxygen, and carbon dioxide supply ; 

 supplemented by observation of morphological characters. 



It has long been recognised that the observed differences 

 in root characters have a real physiological significance, and 

 definite enunciation of this view is found in a descriptive paper 

 by Markle {Bot. Gaz., 191 7, 64, 177-205) on the root systems of 

 desert plants. He remarks on the constant presence, even in 

 deep-rooted forms, of surface roots, and even distinguishes in 

 some instances between " anchorage " and " absorptive " 

 roots. He recognises the impossibility of determining the causes 

 of root variation without laboratory experiments, but expresses 

 the opinion as the result of his observations that root com- 

 petition for soil moisture, especially in arid habitats, is a 

 determining factor. Penetrability of the soil is also considered 

 to influence the form of the root system. The practical impor- 

 tance of these root differences and their probable physiological 

 importance is indicated by Hatton {Journ. Roy. Hort. Soc, 

 191 7, 42, 361-399) in his description of investigations of the 

 paradise apple stocks. He devotes considerable attention to 

 the nature of the root systems, and finds that the botanically 

 distinct types of stock in some instances possess equally 

 distinct types of root system, which are constant even 

 from the young stages of the plants. There are, however, 

 indications that variability of season may influence to some 

 extent the nature of roots produced, and it is recognised that 

 soil differences may also play a part in determining the nature 

 of a root system. The wide range of root characters which can 

 be exhibited by one type of plant is also emphasised by the 



