RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 529 



If the relationship is hnear, then dw/dA will be constant, and the 

 standard formula given on page 527 is derived. 



One other general comment can be made at this point: namely, 

 that none of these earlier v^orkers discussed to what extent a compli- 

 cating factor in the interpretation of seasonal trends was the increasing 

 degree of self-shading as the plants grew taller. For example, reference 

 to the original paper of Kreusler ( 1877 ) shows that two grains of 

 Z. mats were sown every 30 centimeters in rows 50 centimeters apart, 

 and at such a density they would shade one another. 



In 1921 Gregory published a detailed paper on the growth of the 

 cucumber, particularly the expansion of the leaf surface, when the 

 plants were grown at difiFerent times of the year under standard glass- 

 house practice. For the individual leaves the change in area with time 

 was always a sigmoid curve, the shape of which was likened to that of 

 an autocatylitic reaction. On the other hand, the expansion of the total 

 leaf surface was exponential under spring and summer conditions of a 

 high level of light but under December conditions, after following an 

 exponential curve intitially, the rate fell away later. When potted 

 plants were grown in a chamber where the illumination was by means 

 of tungstan filament lamps, the "winter" relationship held, and Greg- 

 ory concluded that this was probably because the temperature (95° F.) 

 was too high. Subsequently ( 1928 ) the experiments were repeated over 

 a range of temperatures from 63 to 90.8° F., and at each temperature 

 the relationship between the logarithm of the leaf area against time 

 was linear but at suboptimal temperatures the rate of expansion was 

 independent of temperature. Gregory emphasized that the increase in 

 leaf surface depended on both the size and rate of expansion of the 

 individual leaves and the number of leaf initials laid down by the 

 apical bud; in consequence, internal or external factors might operate 

 diflferentially on either phase. 



It is of some interest that when Milthorpe (1959) re-examined the 

 factors responsible for determining the rate of leaf expansion in the 

 cucumber, he concluded that Gregory's failure to demonstrate a tem- 

 perature efiPect in the suboptimal range was due to the low intensity of 

 illumination attainable with the then existing light sources. 



In the interval between the two papers on the cucumber, Gregory 

 published a further paper ( 1926 ) on the pattern of dry-matter produc- 

 tion of barley grown in the open under the conditions of pot culture. 

 By frequent sampling, the courses of dry-matter and leaf production 

 were obtained for five experiments covering four years; in two experi- 

 ments the level of added nitrogen was high and in three it was low. In 

 the individual experiments the changes in weight with time followed a 

 sigmoid pattern, and Gregory concluded that this change was best 

 fitted by an autocatalytic curve which was "inherent internally and 



