226 Influence of Environment on Plants 
being concerned with the conditions governing the transformation 
of species. 
lL. THE INFLUENCE OF EXTERNAL CONDITIONS ON FORM-PRODUCTION 
IN SINGLE SPECIES. 
The members of plants, which we express by the terms stem, leaf, 
flower, etc. are capable of modification within certain limits; since 
Lamarck’s time this power of modification has been brought more or 
less into relation with the environment. We are concerned not only 
with the question of experimental demonstration of this relationship, 
but, more generally, with an examination of the origin of forms, 
the sequences of stages in development that are governed by re- 
cognisable causes. We have to consider the general problem; to 
study the conditions of all typical as well as of atypic forms, in other 
words, to found a, physiology of form. 
If we survey the endless variety of plant-forms and consider the 
highly complex and still little known processes in the interior of cells, 
and if we remember that the whole of this branch of investigation 
came into existence only a few decades ago, we are able to grasp the 
fact that a satisfactory explanation of the factors determining form 
cannot be discovered all at once. The goal is still far away. We are 
not concerned now with the controversial] question, whether, on the 
whole, the fundamental processes in the development of form can 
be recognised by physiological means. A belief in the possibility of 
this can in any case do no harm. What we may and must attempt is 
this—to discover points of attack on one side or another, which may 
enable us by means of experimental methods to come into closer 
touch with these elusive and difficult problems. While we are forced 
to admit that there is at present much that is insoluble there 
remains an inexhaustible supply of problems capable of solution. 
The object of our investigations is the species; but as regards the 
question, what is a species, science of to-day takes up a position 
different from that of Darwin. For him it was the Linnean species 
which illustrates variation: we now know, thanks to the work of 
Jordan, de Bary, and particularly to that of de Vries’, that the 
Linnean species consists of a large or small number of entities, 
elementary species. In experimental investigation it is essential that 
observations be made on a pure species, or, as Johannsen? says, 
on a pure “line.” What has long been recognised as necessary in 
the investigation of fungi, bacteria and algae must also be in- 
sisted on in the case of flowering plants; we must start with a 
single individual which is reproduced vegetatively or by strict self- 
1 de Vries, Die Mutationstheorie, Leipzig, 1901, Vol. 1. p. 33. 
* Johannsen, Ueber Erblichkeit in Populationen und reinen Linien, Jena, 1903. 
