The Control of Plant-form 239 



question as to which kind of variation is produced depends on the 

 greater or less degree of alteration; it is correlated with the state 

 of the particular cells at the moment. 



In this short sketch it is only possible to deal superficially with a 

 small part of the subject. It has been clearly shown that in view of 

 the general dependence of development on the factors of the environ- 

 ment a number of problems are ready for experimental treatment. 

 One must, however, not forget that the science of the physiology of 

 form has not progressed beyond its initial stages. Just now our first 

 duty is to demonstrate the dependence on external factors in as 

 many forms of plants as possible, in order to obtain a more thorough 

 control of all the different plant-forms. The problem is not only to 

 produce at will (and independently of their normal mode of life) 

 forms which occur in nature, but also to stimulate into operation 

 potentialities which necessarily lie dormant under the conditions 

 which prevail in nature. The constitution of a species is much 

 richer in possibilities of development than would appear to be the 

 case under normal conditions. It remains for man to stimulate into 

 activity all the potentialities. 



But the control of plant-form is only a preliminary step the 

 foundation stones on which to erect a coherent scientific structure. 

 We must discover what are the internal processes in the cell pro- 

 duced by external factors, which as a necessary consequence result in 

 the appearance of a definite form. We are here brought into contact 

 with the most obscure problem of life. Progress can only be made 

 part passu with progress in physics and chemistry, and with the 

 growth of our knowledge of nutrition, growth, etc. 



Let us take one of the simplest cases an alteration in form. 

 A cylindrical cell of the alga Stigeoclonium assumes, as Livingstone 1 

 has shown, a spherical form when the osmotic pressure of the culture 

 fluid is increased; or a spore of Mucor, which, in a sugar solution 

 grows into a branched filament, in the presence of a small quantity 

 of acid (hydrogen ions) becomes a comparatively large sphere 2 . In 

 both cases there has undoubtedly been an alteration in the osmotic 

 pressure of the cell-sap, but this does not suffice to explain the 

 alteration in form, since the unknown alterations, which are induced 

 in the protoplasm, must in their turn influence the cell-membrane. 

 In the case of the very much more complex alterations in form, such 

 as we encounter in the course ^of development of plants, there do 

 not appear to be any clues which lead us to a deeper insight into the 

 phenomena. Nevertheless we continue the attempt, seeking with the 



1 Livingstone, " On the nature of the stimulus which causes the change of form, etc." 

 Botanical Gazette, xxx. 1900; also xxxir. 1901. 



2 Bitter, " Ueber Kugelhefe, etc.," Ber. bot. Gesell. Berlin, xxv. p. 255, 1907. 



