IN THE MOTOE OKGANS OP LEAVES. 



51 



far differences of a like nature can be recognized in the case of nyctitropic tissues 

 generally. In the case of the guard-cells, it Was easy to show that wo were dealing with 

 elements characterized by their relative youth, functional .strength, and struetural wcakm 

 certain directions; it now remains to ascertain how far similar distinctive features ire 

 >sent in the masses of tissue which make for the diurnal portion in other examples of 



in 



pr 



nyctitropis.n. 



That the masses of tissue which make for the diurnal position of nyctitropic leaves 



are younger than those making for the nocturnal one, is very clearly indicated in must 

 cases by the fact that in tne assumption of the nocturnal position there is a manifest 



tendency 



Inch was a permanent position at a very early stage of 



development of the leaf, or the diurnal one during an earlier portion of tin; period during 

 which movements occur. In the leaves of most spe ies of Jkmhmia immediately before 

 movements begin to occur, the lainime are depressed at angles of various degree* from 

 the line of the petiole and have their two halves folded up, so that their upper mrfaees 

 are in close contact with one another. When nyctitropic phenomena begin t«. appear, the 



lamiine rise and unfold during the day and sink and fold up at sundown (Plate 11 

 Fig. 6). In Cassia a/ata the pinna? just bef >re they become motile, are folded like 



Bauhinia leaves and have their midribs closely applied to the sides of the rachis and 

 directed upwards and forwards at an acute angle to it. When movements set in they 

 consist of diurnal separation of the upper surfaces of the two halves of the lamina; and 

 divergence and depression of the midribs, and of nocturnal folding of the lamina) and 

 convergence and elevation of the midribs. In Cat ia sumatrana the pinna, immediately 

 before they begin to show periodic movements, are disposed as in the previous species, 

 save that their "midribs, in place of being elevated, are deeply depressed beneath the plane 

 of the rachis; and here we have diurnal unfolding of the laminae accompanied 

 elevation and divergence of the midribs, and nocturnal folding coinciding with conver- 



gence 



and depression. In the leaves of Pitlrcolobium samun, which are ultimately 

 _ d by the great development and persistence of their nyctitropic movements, 

 the "permanent position of the various parts at a very early stage of development is as 

 follows. The lower part of the primary rachis is directed at a very acute angle to the 



d the distal part is at first somewhat curved inwards (v«h Plate I, Fig. 13), and 

 bsequently, as the weight of the secondary rachises and pinnules increases, abruptly 

 rved outwards. The secondary rachises are closely appressed to the primary one and 



e directed obliquely upwards, and the pinnules are fully expanded and are related to 



axis 



condary rachises as 



pritnarj 



the latter are to tlie primary one. Somewhat later, as the wood 

 achis straightens out, still retaining an acute angle to the axis. 



The secondary rachises at the same time sink and are ultimately depressed beneath the 

 plane of the primary one, whilst the pinnules retain their original arrangement. When 

 nyctitropic movements set in, they are not complicated by any folding and unfolding of 

 the pinnules, as these are from the outset fully expanded, and the diurnal position is 

 characterized simply by divergence of the primary rachis from the axis, divergence and 



of the secondary rachises, and depression of the pinnules, so that then- surfaces 



ele\ 



come to look upwards and downwards in place of inwards and outwards. The entire 

 series of pinnules on any secondary rachis here, as in the case of many other bipinnate 

 leaves both in their original arrangement and in their movements, clearly corresponds *" 



.£*••<*! «..~- „ 



the lamina of one pinna in a pinnate leaf such as that of a Cassia, whilst in other cases 



we 



find them each originally folded and 



o 



ponding to those of 



Ann. Roy. Bot. Gakd. Calcutta Vol. VI. 



