376 LECTURE XVI. 



variations in the relative rate of growth of opposite sides of 

 the organ, or, to express it in a single word, to spontaneous 

 heterauxesis. This heterauxesis may be accounted for in the 

 same way as the irregularities in the rate of growth in length 

 of the organ as a whole. If the evolution of energy be 

 equally active in all the cells of any given transverse zone 

 of a growing organ, and if the mechanical. conditions be the 

 same in all; they will all grow at the same ^uniform rate, and 

 the organ will not exhibit nutation, but its apex will travel 

 upward in a straight line. When, however, these conditions 

 are not fulfilled, heterauxesis takes place and the organ 

 nutates. 



We will defer for the present a discussion of the intimate 

 causes of spontaneous heterauxesis and will pass on to the 

 consideration of the influence of external conditions in in- 

 ducing variations in the rate of growth. We. have already 

 learned incidentally that certain external conditions have an 

 important influence upon growth. We have learned, for 

 instance (p. 292), that aerobiotic plants do not grow in the 

 absence of free oxygen, nor anaerobiotic plants in the absence 

 of an adequate supply of fermentable material. We have 

 ascertained further that growth will only go on within certain 

 limits of temperature (p. 293), and finally, that one of the 

 essential conditions of growth of an organ is the supply of 

 enough water to maintain the growing cell or cells in a state 

 of turgidity (p. 335). 



TEMPERATURE. 



It was pointed out in a previous lecture (p. 293), that for 

 each plant there is a minimum temperature at which growth 

 is just possible, an optimum temperature at which it is most 

 active, and a maximum temperature at which it is arrested. 

 There remain to be noted one or two points in the relations 

 between growth and temperature which we shall find of 

 importance hereafter when we come to consider the effect of 

 the simultaneous variation of several external conditions, and 

 seek to analyse the complex result. 



