102 PLANT PHYSIOLOGY 



Keeping at first general symmetry before us, we see the regularity of the arrange- 

 ments on which e.ven the position of organs is based. One naturally thinks 

 of the 



1. 49 P. 339, 1. 43, for The relations . . . demonstrated read Again, it is 

 impossible to deal with the longitudinal interrelation of vessels, fibres, and sieve- 

 tubes, without taking into account a mutual influencing of one by the other. 

 The relations existing between the vascular bundles and the leaves introduce 

 us to another set of correlations, and, finally, we meet with the same phenomenon 

 in the relations subsisting between growth in surface and in thickness of the 

 individual cell, and especially when we take into account the correspondence 

 of pits. 



There is yet another reason, not yet alluded to, which cannot be over- 

 emphasized as explaining the significance of correlations in organogenesis. 

 The growing point of the stem is not affected at all, or only to a slight extent 

 directly, by many external factors such as light, water, &c. ; when such do 

 influence the formation of the plant, they can only directly affect full-grown 

 parts ; these must affect in turn the growing point, for the effect of external 

 factors is often already apparent in the bud ; where they come into direct 

 contact with an organ, however, they produce no effect. 



Numberless examples might be quoted, but these must suffice. Sum- 

 marizing our discussion of correlations in a sentence, we might say that the 

 structure of the plant under natural or experimental conditions is not fixed 

 from the beginning, but is essentially determined by correlations existing 

 between the whole and its parts. 



We have divided the factors concerned in plant formation into internal 

 and external ; we have now to inquire in which of these categories the 

 phenomena of correlation are to be included. 



Looking at the plant as a whole the most natural point of view to take- 

 correlations are to be regarded as internal factors ; but when one takes into 

 account the relative independence of the individual cell or bud, the action of 

 other cells or growing points on these may be regarded as external. But even 

 in the single cell of a growing point we have a complex structure we distinguish 

 cell-wall, cell-sap, and protoplasm. We know that the essential formation of 

 a plant depends on the protoplasm, and cell-sap and cell-wall are as much 

 external environment to it as neighbouring cells are to the cell as a whole. 

 Hence, following KLEBS (1903, 1904, 1905), one might assume three causes of 

 plant formation : (i) external factors ; (2) internal conditions ; (3) specific 

 structure. In ' specific structure ', be it of a lifeless body or of an organism, 

 KLEBS recognizes the factor concerned in determining the appearance of a 

 definite peculiarity under definite conditions. Specific structure determines 

 the ' capacities or potentialities of a body '. For instance, as water has the 

 capacity of being solid, fluid, or gaseous, so the species Sempervivum Funkii has 

 the capacity of growing leaf rosettes, offsets, or flowers. The internal conditions 

 at the growing point determine which of these capacities shall become effective. 

 By ' internal conditions ' KLEBS (1905) understands ' the molecular affinities 

 and forces which operate amongst the enormous mass of minute particles which 

 go to make up the cell '. He goes on to say ' each cell is the product of another 

 cell, and possesses, in consequence, from the very commencement certain 

 peculiarities in its internal conditions, in relation to water-content, osmotic 

 pressure, the presence or absence of certain substances, carbohydrates, proteid, 

 acids, salts, &c. All this I include under the term internal conditions, and it 

 depends on the special peculiarities present from the beginning which poten- 

 tiality of the specific structure of the cell will become active, whether it grows, 

 divides, alters its form in any one of many ways, &c.' 



