108 



THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



in 



very dry hot weather, even if they be subsequently kept 



the shade, or in those 



detached in cooler and damper weather 



when exposed to strong direct sunshine afte 



r 



parat 



But if transpiratory loss be slight, the tissues will retain a 



amount of water, especially where atmospheri 



and this, of course 

 elements to a gres 



and tell 



derabl 



humidity have antecedent!} 



been high for some time 

 cities of the active tissue 



available to 



the osmotic capa 



9 



or less extent. Mere separati 



fro 



m 



the axis has in no way affected the functional properties of the active tissue element 

 remote from the site of section, it has on 



pply of 



and, so long as 



removed them from all connection with 

 the inherent water-conducting system retains a 



suffi 



syst 



quantity of water, this is of no importance 



Whilst the 



water-conducting 



is a suffi 

 that a m< 



store of inherent water, it plays the same part to the 



does to a plant as a 



hoi 



Th 



tissues 



pulvini which make for the diurnal position are functionally much strong 



>nents, and hence under continued stimulation by light will undergo a g 



the tertiary 

 than their 



opponents, 



in osmotic capacity than the latter, and as they also present greater st 



for absorption, will appropriate a correspondingly large share of the 



tural facilities 

 store of water 



r 



posal. 'With this they will increase in 



o 



th, and a reversion of the 



pinnules towards the diurnal position must follow. But if, when this reversion has 

 been fairly established, the leaf be suddenly exposed to direct sunshine, a sudden rise 



transpiratory loss of water must at once ensue, and with this a renewed dra 



upon 



leading as before to special 



weakening 



of those portions of the pulvini 



reserve 



fluid 



m 



the water-conducting 



which make for the diurnal position. If the activity of transpiratory loss does not 

 exceed a certain limit, and if the amount of 

 system be high, this will merely lead to slow, progressive alterations in position of the 

 mules, like that which occurs in leaves still attached to the axis when exposed 



pinnules, 



to strong sunsh 



ne. But if transpiratory loss be 

 ithin the water-conducting tissues have been 



J 



pid, and the reserve of water 



mor 



or 



less exhausted in supplying 



the osmotic demands of the 



be 



tablished to g 



ily turgescent tissues, a sufficiently rapid dr 



b 



may 



rise to the occurrence of rapid resumption of the nocturnal 



position. Subsequently, under continued solar stimulation 



ed struggle fo 



any 



and a tendency to 



y 



anifests itself in 



gi 



remaining reserve-water sets in, 



or less degree, never, however, leading to anything beyond very partial resumpti< 



the diurnal position because of the limited amount of water which is available. Fi 



of 



as transp 



continues, the 



ditions for active 



and 



imately abolished, and with this the 



turgescence are gradually diminished, 

 parts of the leaf permanently assume 



the positions which are determined by their passive structural peculiarities 



During continuous periods of fine, settled weather the phenomena attending separati 



of le 



from the axis show regular 



(1 



picuous differences, accord 



o 



of day at which the experiment is conducted. The effect 



of 



primary sepa 



to the time 

 ration from 



the axis 



d of 



ubsequent exposure to cliff u 



d direct sunshine are not alike in the 



arly morning and 



after 



When leaves are separated in the morning, ere the 



dew has evaporated, and whilst the air is relatively humid, the extent and rate of 



ption of the nocturnal position by the pinnules is less than it is in 



ater, the tendency to the occur- 

 is less; secondary partial recovery 



primary assun 



the afternoon, the amount of primary recoveiy is gr< 



rence of rapid movement on exposure to direct sunshine 



from the nocturnal positio 



after 



pid movements constantly occurs in greater or less 





























