Ill DEVELOPMENT AND ENVIRONMENT 69 



scopic examination at intermediate stages shows that the 

 plate of dividing cells is only one cell thick, that the 

 divisions begin at the periphery, and extend towards the 

 centre, and that the dividing wall between the daughter 

 cells splits immediately along its middle lamella. The 

 daughter cells which are left on the face of the scar, after 

 the stalk has broken away, bulge outwards, and form a 

 simple callus. 



In many plants this stimulus is also provoked by non- 

 pollination, so that unfertilised flowers are shed. This 

 occurs in cotton, of course, but it does not seem to be 

 common under ordinary conditions. 



For the present we are unable to form any clear 

 conception of the chain which extends from the severed 

 root to the absciss layer. It is certain that the main 

 factor, if not the only one, is the water-content of the 

 plant. Mere severance of the root does not provoke 

 shedding as a traumatic stimulus ; thus, plants which are 

 screened from direct sun after the root has been damaged, 

 show little or no shedding. Consequent on such root- 

 damage we find a general closure of the stomata, and it 

 seems at least probable that this abnormal closure reacts 

 • first of all on the absciss layer, by bringing about 

 abnormal internal temperatures. Much more experimental 

 "work is required before further discussion can be 

 profitable. 



Though the primary cause of shedding in Egypt is 

 a deficient root-absorption, it follows that an excessive 

 transpiration rate must produce the same result, since the 

 terms " deficient " and " excessive " are relative. A very 

 dry hot day may provoke shedding, but such weather is 

 infrequent at the time when shedding is important. The 

 heavy shedding of wide-sown plants as compared with that 

 of plants in field crop is probably caused by excessive 

 transpiration, or rather, by too great irregularity in this 

 function, since wide-sown plants are freely exposed to wind, 



