342 METAMORPHOSIS 



Seeds of the grass type, which can withstand thorough drying, as a rule 

 retain their powers of germination only for a limited number of years, and such as 

 may do so for fifty or more years must be considered as exceptional. What the 

 loss of germinating power depends on in the long run is not known ; but when 

 we reflect that gradual alteration has been proved to take place in the proteid 

 reserves present in dry seeds tending to reduce their solubility, we may conclude 

 that specific protoplasmic bodies undergo as time goes on alterations calculated 

 to render them functionless. At all events it is quite out of the question to 

 suppose that death of the resting seed is brought about by using up of reserve 

 substances in respiration. 



But we must not assume that chemical changes in the interior of the seed, 

 independent of respiration, alone are necessary to explain the ultimate death 

 of the dried seed ; we must also bear in mind the fact that many seeds immediately 

 after reaching maturity are unable to germinate and only show power of develop- 

 ment after a certain period of hibernation. Thus, according to Kienitz (1880), 

 the seeds of the ash, hornbeam, and of Piniis cembra begin to germinate the year 

 after they ripen, and it is known in the case of other plants that individual 

 differences occur among the seeds themselves (Winkler, 1883) ; thus seeds of 

 Euphorbia cyparissias germinate in the course of four to seven years. It is now 

 definitely known that these variations depend on varying degrees of permea- 

 bility of the testa for water, but we know nothing further as to why seeds which 

 have imbibed water are prevented from germinating (Wiesner, 1902, p. 55) ; at 

 most we may draw analogous conclusions from the behaviour of resting vege- 

 tative buds, a subject of which we shall have to speak later on. Undoubtedly 

 internal factors play the chief part in determining the initiation and cessation 

 of the resting period in seeds, whilst the hibernation of lichens and mosses would 

 appear to depend entirely on external conditions. 



If the seed be provided with the external and internal conditions necessary 

 for germination, the plant resulting from its development exhibits numerous 

 periodic phenomena, which are partly autonomous, partly dependent on its 

 relation to the environment, e. g. daily and yearly periodicities. 



Thus we may observe a daily periodicity in longitudinal growth, which 

 is perfectly intelligible if we remember that plants are subjected to certain 

 external factors which actively affect growth and which themselves vary peri- 

 odically ; thus we have a daily periodicity in illumination, in heat, and amount 

 of atmospheric moisture. These factors act, however, so unequally and even 

 antagonistically that it is impossible to calculate their combined effect before- 

 hand. Even if we omit from consideration atmospheric moisture, an in- 

 crease in which as a general rule accelerates growth, and confine our attention 

 to light and heat, we still find that changes in these two factors may have the 

 most varied results ; the plants, in a word, may grow more rapidly either by 

 day or by night. On a midsummer day a high temperature, approaching the 

 maximum point, in conjunction with bright light retards growth, and increased 

 growth in the evening may be due not merely to darkness but also to cooling 

 down to somewhere near the optimum temperature. Conversely, in springtime 

 the great reduction of temperature at night limits growth so much that the 

 maximum growth occurs by day owing to the higher temperature, in spite of 

 the retarding effect of light on growth. 



In the experimental treatment of this question, where two variables are 

 under investigation, we must naturally deal with them one at a time. Ex- 

 periments in this direction have been carried out by Sachs (1872) and Godlewski 

 (1898, 1890), who studied the effect of variations in light intensity under constant 

 temperature and moisture conditions. Sachs found that the rate of growth of 

 the stem reached a maximum in early morning after sunrise, that it decreased 

 hourly towards evening, and increased once more as darkness came on, often 



