278 BOTANY PART i 



SECTION II 

 DEVELOPMENT ( 52 ) 



DEVELOPMENTAL PHYSIOLOGY, which is also spoken of as the 

 MECHANISM OF DEVELOPMENT, will be treated here under three heads. 

 A few introductory remarks will in the first place render more vivid 

 some facts that have already been mentioned in the morphological part. 

 On this follows developmental physiology in the proper sense, the 

 object of which is to understand causally the successive processes in 

 development arid to modify these at will. As yet the results 

 obtained do not reach far towards Uiis goal ; the problems are more 

 numerous than the solutions. These problems require to be presented 

 from two points of view : in the second sub-section the factors which 

 influence development will be considered, while in the third sub-section 

 the presentation will be based on the developmental processes them- 

 selves. 



I. Introductory Remarks 



Development accompanied by changes of form due to growth is 

 one of the most general and striking of the vital phenomena of the 

 plant. A mere increase in volume does not necessarily imply growth, 

 for no one would say that a dried and shrivelled turnip grows when 

 it swells in water. Only permanent and irreversible increase of size 

 can be termed growth, and this whether the plant as a whole is gain- 

 ing or losing in substance. Usually growth is associated with gain of 

 material, but in the case of potatoes sprouting in a dark cellar loss 

 takes place by transpiration and respiration, and yet the shoots 

 exhibit growth. 



1 . The Measurement of Growth. 



Total Elongation. The rate of growth of a plant, or the total 

 elongation in any unit of time, may be directly measured by means of 

 a scale in the case of some quick-growing organs, e.g. the inflorescences 

 of Agave and the shoots of Bambusa. Usually it is necessary to magnify 

 in some way the actual elongation for more convenient observation. 

 This may be effected by means of a microscope, which magnifies the 

 rate of growth correspondingly with the distance grown. For large 

 objects, the most convenient and usual method of determining the 

 rate of growth is by means of an AUXANOMETER. 



The principle of all auxanometers, however they may differ in construction, is the 

 same, and is based upon the magnification of the rate of growth by means of a 

 lever with a long and short arm. In Fig. 255, at the left, a simple form of auxano- 



