

r 



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V THE MOTOR OBGAXS OF LEAVES. 53 



the pulvinar tissue makes for convergence and the distal parts of the leaf for diwi 

 Under these circumstances, it need not surprise us to find conspicuous changes in the 

 position of the leaves of Mimosa taking place under the influence of factors nffeetii 

 pulvinar tnrgesoenoe which are incapable of producing any appreciable results in the ca 

 of the leaves «»f Pith^cohbium. 



h\ some cases, as in the pinnules of P i Ihccohbiutn saman and Leuctrna qlavca tlie 



movements during a great part of the lifo of the leaf effect a complete nocturnal restora- 



tion of what was a permanent position at an early stage of evolution, and when they 



to do so, determine a simple incomplete restoration; but in other crises, as time ir» 

 e development and increased strength of certain masses of tissue introduce complicn- 

 and we no longer find simple complete or incompleto restoration taking plan In 



the pinnules of Pithecolohium and Lrucwna glauca we are dealing With lamhue which 



from the outset are fully unfolded, and the movements which they exhibit are simply d 





to the subsequent development of a mass of parenchyma over the insertion and base of 

 the midrib, which, when its turgescence rises under the influence of solar stimulation, 



is able to determine the divergence and depression of the lamina. JJut in cas< >, such as 



pinna? of Cassia alaia where the lamina is primarily folded and shortly alien ml 



becomes permanently expanded, the characters of the movements become modified when 



the latter condition has been » Mablished, factors which previously determined expansion or 

 folding of the lamina now coming to determine movements of rotation in it. This 

 modification of movement may be more satisfactorily dealt with in considering the 



particular phenomena presenting themselves in individual leaves, and i- merely alluded 



to here in order to indicate that wo may not in all cases be able at once to recognize that 



ctumal position corresponds to 



lly permanent position. Jn 



tponds to a tendency to such reversion, but the degree and simplicity 

 with which the process is earned out is often greatly affected by structural modification. 

 which have been established subsequently to the period at which the primary permanent 

 position was first departed from. 



The fact that the masses of tissue making for the diurnal position of the various 

 parts of nyctitropic leaves, agree with the corresponding elements determining the diurnal 

 condition of the stomata in being relatively young, is thus indicated by the characters o 

 the movements which they give rise to; but we are fortunately not obliged to remain 

 satisfied with evidence of this nature, as in many instances we find most conspicuous 

 structural indices of the same fact. Figures a to g, Plate I, Fig. 11, show the appear- 

 miam presented by the primary pulvini of seven successive leaves on a .shoot of Pithecob- 



bium saman, and in passing from 1, which is the youngest, to 7, which is the most 

 mature member of the seri< 3, the excessive development of the mass of tissue on the 

 upper aspect of the pulvinus during the later stages of evolution comes out most 

 distinctly. The evidence afforded by sections through the thickness of pulvini of different 

 io-es is equally convincing, for these show that at first the development of the superior 

 pulvinar pad lags behind, and at a later stage overtakes that of the inferior one. 

 (Plate I, Fig. 15; Plate IV, Figs. 1, 2.) So, in the case of the tertiary pulvini, th< 

 superior pad is at first very inconspicuous, but ultimately appears as a prominent 

 mass of vivid green tissue. In cases where, as in the primary pulvini of Cassia alata, 



the fibr 



does not a sume the form of a simple unlignificd central cord 



but remains arranged in a ring and undergoes nonnal processes of lignificat 

 conspicuous periodic movements do not manifest themselves, and the continued gro 



