332 The Living Plant 



When, now, we inspect this graph somewhat closely we find 

 its most remarkable feature to consist in its great irregularities; 

 and the same thing appears in any others, from whatsoever source 

 they are taken. In other words, the growth of plant-structures is 

 extremely irregular in rate. It will not take the reader very long 

 to ascribe the irregularities to the real cause of the most of them, 

 namely, — variations in the external conditions of temperature, 

 light, moisture and so forth. In order to determine the precise 

 effect of each of these conditions, it is only necessary to plot the 

 simultaneous graphs of temperature, moisture, and light, ob- 

 tained as already described under Transpiration, upon the same 

 sheet with the growth graph; and this has been done in the ex- 

 ample presented above (figure 123). This subject of the effect of 

 external conditions upon growth is, however, so important, that it 

 must be considered somewhat farther. 



First, as to the effects of temperature upon growth. Every- 

 body knows, in a general way, that plants grow faster in warm 

 weather and slower in cold; and in the early spring we see ample 

 illustration thereof in the way the grass comes up fastest in the 

 warmest corners, or in places where warm pipes, such as sewers 

 from houses, cross lawns, — marking their courses by the early 

 greenness above them. In our graphs the reader can see how 

 closely the rise and fall in growth rate is connected with the rise 

 and fall of the temperature. The same thing is shown, and very 

 much clearer, by an instrument, devised for the purpose, and 

 shown in our figure (figure 124). It must suffice to say that by 

 its aid one can determine in a continuous band of soil the lowest 

 temperature at which a plant can be made to grow (the minimum) , 

 the temperature at which it grows its very best (the optimum), 

 and that above which it will not grow at all (the maximum). 

 Between the minimum and maximum, the tips of the growing 

 plants plot, as it were, their own curve of the relation of growth 

 to temperature, culminating at the optimum, as our picture well 

 shows. 



