THE WATER FACTOR IN GROWTH 607 



14 hours of illumination per day, while the top received 9 hours and was kept 

 exflorated, flowers developed only on the base. 



The results of a recent investigation by Zimmerman and Hitchcock (1936) 

 also point strongly toward a hormonal mechanism for at least some types of 

 photoperiodic effects. When Jerusalem artichokes developed under long-day 

 and short-day conditions during the summer months the former produced 

 underground stems but no tubers, the latter tubers but no underground stems. 

 If, however, the stem tips only were subjected to short-day conditions by 

 covering them with black cloth caps, while the rest of the plant received the 

 normal summer day-length, they behaved as if the entire plant was subjected 

 to short-day conditions, i.e. produced tubers. Obviously differences in response 

 of the stem tips to different photoperiods are in some way communicated to 

 the underground organs of the plant and there exert a regulatory influence 

 upon their development. Such an effect can be most easily visualized in terms 

 of a hormonal mechanism. 



Important practical applications of the principles of photoperiodism have 

 been made, especially to the growing of floricultural greenhouse crops. Short- 

 day species such as chrysanthemums can be brought into bloom earlier in the 

 fall by decreasing the length of their daily exposure to light. Most flori- 

 cultural species, however, are of the long-day type. The time required for 

 the majority of such species to attain the flowering stage can be shortened 

 during the winter months, often very markedly, by increasing the day- 

 length with supplementary artificial illumination. This procedure has been 

 found to be commercially practical with a number of species (Laurie and 

 Poesch, 1932). 



The Water Factor in Growth. — The dynamic condition of the water in 

 a plant is largely controlled by the opposing effects of the processes of trans- 

 piration and water absorption as already described in Chap. XVIII. When- 

 ever the rate of the former process exceeds the latter for any appreciable 

 period of time the volume of water within the plant shrinks. This results 

 in a diminution in cell turgidity, an increase in the diffusion pressure deficit 

 of the water in the cells, and a decrease in the hydration of the protoplasm 

 and cell walls. A decrease in the hydration of the protoplasm in the cells 

 of any meristematic tissue invariably results in a cessation or checking of one 

 or more of the phases of growth. 



Contrariwise, a shift in environmental conditions which brings about, di- 

 rectly or indirectly, an increase in the hydration of the protoplasm of a 

 meristem usually results in an increased rate of growth if no other factors 

 are limiting. 



The water factor as it affects growth processes is primarily an internal 



