288 BOTANY PART i 



of the " permanent tissues " from the primary and secondary meristems 

 has been described in the morphological section. Here it is only 

 necessary to recall the fact that the following processes are con- 

 cerned ; in the first place the formation of cells by cell division which 

 takes place in the embryonic tissues and at the commencement of 

 elongation ; following on this the separation of cells which gives rise 

 to intercellular spaces ; the independent growth of the individual cells ; 

 thickening and chemical changes of the cell walls ; modifications (and 

 eventually in some cases the complete disappearance) of the cell 

 contents ; and lastly fusions of cells (cf. p. 44). 



In the arrangement of the tissues the same symmetry, which is 

 apparent in the external form of the organs of the plant, is seen ; the 

 internal structure of the organs is thus radial, bilateral, or dorsiventral. 



II. The Factors of Development 



In attempting to determine the factors which influence development 

 it is necessary to treat of examples which show in characteristic fashion 

 the effect of particular factors. Completeness, either in the enumera- 

 tion of the factors or as regards their influence, is out of the question. 

 It is advisable to select the simplest influences when possible, since 

 more complicated cases require further investigation. As in other 

 cases the factors may be divided into the two groups of external and 

 internal factors. 



A. External Factors 



All the forces and substances which have been seen to be 

 physiologically effective in the metabolism, or which play a part in 

 movements, are among the external factors of development. 



Certain external factors were mentioned on p. 218 as general 

 conditions of life ; without these it is evident that no development 

 would take place. These, and other factors which are not necessary, 

 exert a profound influence on growth. Quantitative and even quali- 

 tative changes in the organs of plants may be educed by variation in 

 the intensity, quality, or direction of such factors. These influences, 

 in which the connection between cause and effect is always compli- 

 cated and involves stimulation of the protoplasm, are termed 

 formative. 



1. Temperature. As in the case of metabolism it is found that 

 a certain temperature is a necessary formal condition of growth. 



There is complete cessation of growth at a temperature less than 

 or higher than 40-50. Between the MINIMUM and MAXIMUM 

 temperatures, at which growth ceases, there lies an OPTIMUM tem- 

 perature at which the rate of growth is greatest. This optimum 



