296 BOTANY PART i 



plant frequently reacts in a purposive fashion will not be considered 

 here (cf. p. 212). 



B. Internal Factors 



When a change occurs in an organism while all the external factors 

 remain constant it must be referred to internal factors. The latter 

 cannot be so readily analysed as the external factors, so that the 

 reference of many phenomena to internal factors is frequently little 

 more than a statement of our ignorance. 



1. Determinants. The determinants which a plant has derived 

 from its parents are the first internal causes to be mentioned ; it is 

 these that lead to the regular origin of a fungus from a fungal spore 

 or of a bean-plant from a bean-seed. In particular they determine the 

 agreement of all the individuals of any species, when under the same 

 external conditions, in such characters as the colour of the flower, 

 form of the leaf, size, etc. It is not as a rule possible to experimentally 

 alter the determinants possessed by a species, and they cannot be 

 ascertained by direct observation On this account further considera- 

 tion of them may be deferred until heredity is treated later. 



2. The Phenomena of Correlation ( 61 ). While external factors 

 have a profound influence on the internal structure of plants the 

 differentiation of tissues proceeds under quite constant external con- 

 ditions ; it is thus determined by internal causes. We do not know 

 what is the nature of the particular causes that force a meristematic 

 cell into a definite course of development. Only one thing is certain ; 

 from every cell of the growing point everything might arise, all the 

 cells agreeing in their determinants. It is the mutual connections or 

 correlations between the cells that lead to the lines of development 

 followed by this and that cell. When these connections are removed 

 it has been seen in the phenomena of reparation (p. 282) how cells 

 exhibit quite other capacities than those they had previously shown 

 when in connection with one another. This applies to mature as well 

 as meristematic cells when their connection with neighbouring cells is 

 interfered with. Thus in the process of regeneration (p. 282) it has 

 been seen how fully-grown cells that would soon have perished again 

 become young, and how, for example, from a single epidermal cell all 

 the various cells characteristic of the particular plant can be derived. 

 It is clear that an organism in which such mutual action of the cells 

 was lacking could not exhibit the division of labour that is customary 

 in the higher plants. In other words, correlations must be reckoned 

 among the " regulations " without which the organism is inconceivable. 



Such correlations exist between the externally visible organs of 

 a plant as well as between its cells. This, if not as a rule evident, 

 becomes apparent when an organ is removed and the reactions of the 

 isolated organ and of the plant from which.it was taken are studied, 



