EXTERNAL CAUSES OF GROWTH AND FORMATION. I 303 



Just as extreme variations occur in the position of the maximum and 

 minimum of illumination, both in the case of individual plants and individual 

 organs, so also it is impossible to make any general statement as to the position 

 of the optimum. 



In nature, all the organs of a plant which are sensitive to light are subject to 

 frequent variations of light intensity, not only owing to the periodic alternations 

 of day and night, but to other causes as well, and it may be experimentally 

 shown that a variation of light intensity often, but not always, influences the 

 rate of growth not only in organs which require a certain amount of light for 

 their development, but also in those which can grow in complete darkness. This 

 fact becomes very obvious if we compare the rate of growth in darkness with 

 that taking place under feeble illumination. We may take a few examples 

 first of all from organs capable of growth in continuous darkness. Stame- 

 ROFF (1897) determined the increase in growth by means of a micrometer in the 

 following structures, which, under constant temperature, were exposed to the 

 light of an electric lamp and darkened every 10-15 minutes alternately. 



Time of 



exposure. Dark. Light. Dark. Light. Dark. Light. Dark. Light. Dark. 



Mncor vegetative cells 10' 777 7 7 77 7 7 



,, conidiophores 15' 10 9 95 8-75 925 8.5 925 825 — 



Marchatttia, roothairs 10' 6 45 6-25 45 625 45 625 4 — 



Robim'a, pollen-tubes 15' — 6 6 6 6 6.5 6 6 6 



The growth of pollen-tubes and of the ordinary hyphae of Mucor was by ex- 

 periments of this kind shown to be unaffected by light, while the conidiophores 

 of Mucor and the rhizoids of Marchantia often exhibited, as a result of illumina- 

 tion, a very considerable diminution in growth. Similar retardations due to light 

 have been shown to occur in stems, leaves and roots of the higher plants by 

 Sachs (1872), Prantl (1873), Strehl (1874), and Kny (1902), Kny compared 

 roots which had been exposed to lights of varying intensity with those which 

 had been kept in the dark continuously for the same time, and found that after 

 the lapse of several days the illuminated roots were markedly shorter than 

 those which had been in darkness. Most of the authors mentioned above have 

 compared — under constant temperature conditions — the increase in growth 

 taking place during the night with that occurring by day. The influence of 

 brief alternations of darkness and light in Phanerogams has been studied by 

 Reinke (1876), who found that the hypocotyl of Helianthus exhibited in every 

 quarter of an hour (in /a) the following increments : — 



This diminution of growth in consequence of light, as already noted, may 

 amount in individual cases to a complete cessation of growth, and a sufficiently 

 great intensity of light may in the long run produce this result in every plant. 

 Generally speaking, however, ordinary daylight, if it be acting continuously, 

 produces merely a retardation and not an arrest of growth. Observations 

 made on plants in the arctic regions, as well as the experiments which Bonnier 

 (1895) has carried out in our own latitudes with artificial light, prove this con- 

 clusively. 



It does not necessarily follow that the final shape and size of the plant 

 should be influenced by the accelerating and retarding stimuli which we have 

 hitherto dealt with ; but as a matter of fact they are often actually so affected, 

 and thus we may speak of a formative influence of light. At the same time we have 

 to discriminate between the intensity, the direction, and the quality of the light 

 which falls on the plant [especially the distribution of light on its upper surface]. 



