EFFECT OF EXTEBNAL AGENCIES ON GROWTH. 611 



early morning, and is at its minimum about 2 P.M., from which time it 

 gradually increases. In annual plants the maximum degree of tension is 

 at the base of the stem, and diminishes towards the end of the branch e 

 and of the roots, where the growth is most active. The effect of increased 

 tension at the base of the stem would force the nutrient fluids in the two 

 directions where it is most required. The greatest tension in young shoots 

 is also at the base, the sap being thus forced towards the growing-point. 

 In the case of bulbous plants the greatest tension is exerted in the stem, 

 so that the nutritive juices may be forced into the bulb. 



Periodicity of Growth. Connected with the growth of plants two sepa- 

 rate phenomena are observed : the one a regular onward course of growth 

 from infancy to maturity, varying in degree, rapidity, and duration, accord- 

 ing to hereditary endowments, attaining a maximum, and then gradually 

 decreasing ; the other a fluctuating, intermittent, or periodic growth. The 

 former is called by Sachs the grand period of growth. The rate of increase 

 in the case of the grand period is not directly in connexion with changes 

 of temperature and other outward conditions, but is to some extent inde- 

 pendent of them. The rate of growth, moreover, varies in different parts 

 of the same organ, so that there is one portion where a maximum rate of 

 growth is observed. According to Sachs, the maximum rate of growth 

 in stems is greatest at some distance from the apex of the stem, while in 

 roots the fastest growth takes place much nearer to the apex. But this 

 statement appears, from Bennett's researches on Vallisneria and the Hya- 

 cinth, to be too general. The author last cited found that in the peduncle 

 of Vallisneria the rate of growth was much more rapid near the summit, 

 while in the case of the flower-stalk of the Hyacinth the reverse was the 

 case. In Tritoma the greatest and most rapid growth was observed to 

 take place near the upper end of the scape. While the grand period of 

 growth, considered as a whole, augments in intensity gradually till it 

 reaches a maximum and then declines, yet, if the growth of internodes, or 

 short intervals, of the stem be measured at frequent intervals, the rate of 

 growth is found to be very irregular, so that if projected in a diagram 

 it would not form a uniform curve, but a series of zigzags. These varia- 

 tions Sachs attributes to differences in temperature and other outward 

 circumstances, and further points out the existence of a direct relation 

 between the rate of growth and the tension of the tissues, the curves 

 representing the two phenomena being identical. Reinke, however, dis- 

 putes these conclusions and attributes the variations to inherent changes in 

 the plant itself in association with variations in the degree of humidity, 

 &c. &c. 



Day and Night Growth. The evidence as to whether the greatest 

 amount of growth takes place by day or by night is still conflicting, though, 

 from what has been before said as to growth and cell-division, as well as 

 from direct measurements, the balance of evidence goes to show a greater 

 relative growth in the hours of darkness, the maximum being observable 

 just before sunrise, the minimum soon after noon. The conflicting state- 

 ments arise from the great variations in temperature, light, moisture, &c., 

 so that it is only by eliminating, so far as possible, the effects produced 

 by heat from those which are the result of light, and so on, that a deci- 

 sion can be arrived at ; and when such a decision has been obtained, it is 



