April 1892.] THE BOTANICAL SOCIETY OF EDINBURGH. 421 



Let US suppose that a primitive cell consisted of an 

 aggregation of similar plasmic molecules, i.e., there being no 

 specialisation of any one molecule or groups of molecules ; 

 then it is highly probable that in such an aggregate of iden- 

 tical molecules, should they l)ecome interdependent, special 

 molecules or group of molecules would become modified in 

 certain special directions, according, as owing to relative 

 position, they are differently affected by environmental con- 

 ditions, such as food-supply, light, heat, &c. 



If now the various functions of each of the original 

 unmodified molecules be represented by, say. A, B, C, D, E, 

 then specialisation in one organ might be represented by 

 A, B, C, D, E, and that in another organ by A, b, c, d, E, 

 and so on. 



Further, if we suppose that a group of molecules least 

 exposed to environmental conditions (i.e., a group probably 

 of central position) does not undergo functional specialisa- 

 tion, but simply benefits by the specialisation of the other 

 groups of molecules, then such a centre, owing to its non- 

 specialisation and entire dependence on its neighbours, will of 

 necessity be influenced by all the changes which take place 

 in the cell, changes which will be either directly beneficial or 

 injurious to its welfare. It is this necessity which has 

 led the non-specialised portion to become a trophic centre. 



In a normal unicellular organism the demand for, and the 

 supply of, food will tend to balance one another, the amount 

 of food taken up depending directly on the hunger of the 

 trophic centre and the other plasms of the cell, hunger 

 being an unsatisfied affinity of one element or a group 

 of elements for another element or group of elements. 



"When this desire for food is being satisfied, then new 

 molecules, in some way, arise in the cell identical with those 

 already existing. This increase in the number of molecules 

 must lead to an increase in the bulk of the cell ; bub as soon 

 as a certain size has been reached, then factors unfavourable 

 for a ready assimilation of food, will make their appearance, as 

 Leuckart-Spencer have pointed out, for whereas the surface 

 increases in only two dimensions, the cell increases in three. 

 The first plasma to be affected will be that furthest removed 

 from the food supply, i.e., the trophic centre, which, finding 

 it impossible to get the necessary nourishment, will start the 



