THE BIOLOGY OF PLANT GROWTH 443 



a\'ailability of soil moisture, by limitation in one or another essential 

 mineral, and so on. We know, for example, that a brief period of water 

 stress during the middle of the day drastically reduces the photosyn- 

 thetic rate, and we know, too, that water stress or mineral deficiency 

 slows the rate of growth of plants and increases the length of time re- 

 quired for a crop to effectively cover an area of ground with the 

 numerous layers of leaves needed before full photosynthetic efficiency 

 can be attained. Nevertheless, we know in principle how to conduct our 

 culti\ation of plants so as to raise the over-all efficiency during the 

 growing season to the level of 2 to 3 per cent, and this level is in fact 

 now achieved over small portions of the earth, as in Japan, Denmark, 

 and Holland. 



Our considerations above have applied to the efficiency of plants 

 during the growing season. In a second sense, the growth of plant life 

 on our planet is determined by climate. The distribution of plants is 

 limited by cold, heat, and drought— extremes of climate which deter- 

 mine the length of time over which our plant can usefully absorb light- 

 energy to be stored in the form of plant material. Because of the para- 

 mount importance of temperature in' determining the length of the 

 growing season and the total yield of the plant, a vast amount of enter- 

 prise has gone into determination of the temperature requirements and 

 tolerances of individual plant species. We know for many kinds of 

 plants the optimal day temperature, the optimal night temperature, and 

 the limits of temperature over which the plant can succeed (Went, 

 1957). Let us now ask ourselves, howexer, how does the plant know 

 what the temperature is? What goes wrong with a plant grown in an 

 unfavorable temperature? How does a plant sense and respond to 

 varying temperature? A modest start in the understanding of these 

 matters has been made, and it has turned out that in some cases, at 

 least, the way in which plants sense temperatvu'e has a not-overly- 

 complicated chemical basis. 



Let us take, for example, the cosmos plant. We know the optimal 

 temperature for this species. We ma\- grow it also at an unfavorably 

 lower temperature— a temperature such that our cosmos plant accumu- 

 lates dr>- \\eight onl\- one-half as rapidK- as it does at its optimal tem- 

 peratiue. Cosmos plants can, however, be cured, as it were, of sub- 

 optimal temperature by application to them of small amounts of a 

 single chemical substance, thiamine (Bonner, 1943). Such applications 

 cause cosmos plants grown in low temperatures to behave as though 

 grown in a higher temperature. Applications of thiamine to plants 

 grown at their optimal temperature are without effect. In this case it 

 has been possible to show by direct analysis that plants grown at sub- 

 optimal temperature are characterized by a lower concentration of thi- 

 amine in their tissues than that characteristic of plants grown at the 



