CONSIDERATION OF PLANT GROWTH 185 



in four-gallon crocks of soil with a known moisture content. 

 The crocks were weighed frequently and water was added to 

 maintain: in the first crock, the optimum amount; in the 

 second, the minimum amount that would support continu- 

 ous growth; in a third, the optimum was kept for the first 

 half of the growth period and the minimum for the second 

 half; and in the fourth crock, the first half was grown at a 

 minimum and the second half at the optimum moisture. 

 Only the first and second conditions will be explained. The 

 photograph shows in general the nature of the growth and 

 the bottles for adding the water. The plants with optimum 

 water reached a length of ten feet and were trained along the 

 roof of the greenhouse but are hanging down to the crock in 

 the photograph (Plate XVI). The plants with minimum 

 moisture were less than one-fourth as long. The dry weight 

 of the tops and the surface area of the leaves had the same 

 relative relationship. The number of the stomata did not 

 follow so clearly the one to four relation. The plants with 

 optimum moisture had about ninety thousand stomata per 

 square inch and those with minimum moisture about a third 

 more. It is generally believed that plants growing where 

 moisture is limited have fewer stomata, as was true here on 

 a per plant basis, but they actually have more of them in a 

 given area. This has been found to be true on other plants. 

 If the increased size of the leaves with the moist soil is con- 

 sidered the number per leaf and per plant is much greater 

 than with the dryer soil. The humidity of the air also must 

 have a marked influence on growth but no careful experi- 

 mental work with this condition is known to the writer. 



Perhaps next to temperature and water supply, the sup- 

 ply of food, both as minerals from the soil and as carbohy- 

 drates of photosynthesis, is most important in growth regu- 

 lation. Second only to water the food supply is modified 

 with greater ease and more striking results in the average 

 garden than the other growth factors. Experiments of inter- 

 est will suggest themselves to many gardeners. Each of us 



