222 Relatione hetwecn Groirtli and /'Jnvi'romnent 



tlie mean tein])P7ature fall as low as IT)''', and for three weeks the average 

 niaximuni was above 90° F. The plants were unable to stand the strain 

 and after seven weeks growth theywere dead, having made only 28 gms. 

 dry matter compared with 39 gms. produced during a similar period in 

 series M more than two months earlier. The mean minimum temperatures 

 were comparatively high, the lowest mean being equal to the highest 

 recorded at any other period. Evidently constant high temperature 

 combined with excessive insolation under glass are inhibitory to growth, 

 the best results being obtained with rather less sunshine and a greater 

 range of temperature. 



Autumn plants started in September and October resembled those 

 grown in the winter, with a certain amount of extra growth due to a 

 rather more generous supply of sun and heat. 



It is thus seen that the possible amount of growth depends directly 

 upon the sunshine and temperature at all periods of the year, but that 

 beyond a certain point these beneficial factors become harmful and result 

 in the premature death of the plants. Usually the two factors work 

 together but when one factor, sunshine, remains constant, temperature 

 is able to act independently in influencing growth. The converse of this 

 may also be true but has not been demonstrated in these experiments. 



(6) Relation of the efficiency index to the time of year and 

 the age of the plant. 



The "efficiency index" represents the rate per cent, at which fresh 

 material is continuously added to the plant over a definite period^, 

 and it provides a very useful means of indicating the rate of growth at 

 different periods of the life of a plant. In Fig. the efficiency indices 

 are plotted for weekly periods for all the series. As has already been 

 stated during germination the seedlings lose weight by respiration, so 

 that at the very beginning they weigh less than the seeds from which 

 they came, and during the early part of the first growth period a further 

 loss occurs until assimilation is in full play. At these stages the efficiency 

 indices are negative, but when growth has fairly set in they are always 

 positive, except occasionally at the very end after active growth has 

 ceased but respiration is still continuing. 



The curve shows at a glance that during the summer months, while 

 growth is rapid, the efficiency indices reach a high level; in the spring 

 and autumn they are medium, whereas in the depth of the winter they 

 are very low and indicate that little more growth is made than will 

 provide material to keep the plant alive and progressing very slowly. 

 ' Bljickniiiii. V. II. (1919). "Conniouad Interest Law." Ann. Hot. xwiir. |)p. :{.'»:! :{(iO. 



