132 GROWTH 



bacterized peat grew taller than untreated plants. This 

 aqueous extract contained 20 mgs. of organic matter of which 

 only I *9 mg. represented nitrogen. 



Possibly auximones are connected with the synthesis of 

 complex nitrogenous molecules, for their action on the nitrogen 

 cycle organisms is to increase the rate of nitrogen fixation and 

 nitrification and to depress the rate of denitrification. There 

 is no doubt that the use of bacterized peat may give marked 

 positive results in pot cultures, but to what extent the treat- 

 ment is advantageous to field crops is doubtful.* 



HORMONES. The study of regeneration, correlation, 

 polarity, and cognate subjects f leads to conclusions in some 

 respects indefinite in that no tangible factor is discoverable that 

 will account for the beginning or for the control of certain phe- 

 nomena. A cambium cell divides ; the daughter cell destined 

 to become a permanent tissue element will develop into a 

 phloem element if cut off on one side and into an xylem ele- 

 ment if cut off on the opposite side ; what is it that determines 

 the fate of the cell ? The leader of a spruce is negatively 

 geotropic, the lateral branches are diageotropic ; if the leader 

 be removed, a lateral branch from the topmost whorl will 

 change its habit, become negatively geotropic and carry on 

 the functions of the leader. Why must the leader be removed 

 before a change in tone of a plagiotropic shoot can be effected ? 

 The primordium of a lateral bud is laid down and, apparently, 

 all conditions are favourable for development, yet the bud 

 remains dormant until, say, the apex of the main shoot is re- 

 moved, then the bud will immediately start its development 

 It is true that in many cases the diversion of food will account 

 for the subsequent phenomena but in other instances such an 

 explanation is inadequate and in such examples the question 

 is : What is it that presses the trigger ? 



The subject properly is beyond the scope of an introduction 

 to the physiology of metabolism of plants but the introduction 



*See Russell: " Journ. Board Agric.," 1917, 24, n. 



tSee Bohn : " Compt. rend. Soc. biol.," 1918, 8l, 220. Farmer: "New 

 Phytol.," 1903, 2, 193, 217. Goebel: "Biol. Zentrbl.," 1916, 36, 193. Lang: 

 "Brit. Ass. Rep.," 1915, 701. Loeb : " Bot. Gaz.," 1918, 65, 150; "Journ. 

 Gen. Physiol.," 1919, I, 337. 



