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! 
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, 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 xxx11. 1901. 
2 Ritter, ‘‘ Ueber Kugelhefe, etc.,” Ber. bot. Gesell. Berlin, xxv. p. 255, 1907. 
