676 LEroitT— 1891. 



class Le considers a small one; he states, indeed, tliat 'nearlj^ all, especially the 

 clearly perceptible circumnutations,' are comLined movemeuts belonging to the 

 second of the above categories. 



Although I have perhaps no right to such an opinion without repeating 

 Wiesner's worli, yet I must confess that I cannot give up the belief that cireumnu- 

 tation is a widely-spread phenomenon, even though it may not be so general as 

 we supposed. 



If, then, circumnutation is of any importance wo are forced to ask what is its 

 relation to growth-curvatures. It was considered by my father to be ' the basis 

 or groundwork for the acquirement, according to the requirements of the plant, of 

 the most diversified movements.' ' lie also wrote : - ' A considerable difficulty in 

 the way of evolution is in part removed, for it might be asked how did all these 

 diversified movement.^ . . . . first arise .'^ As the case stands, we Imow that there 

 is always movement in progress, and its amplitude, direction, or both, have only to 

 be modified for the good of the plant in relation to internal or external stimuli.' 



Those who have no belief in tlie importance of circumnutation, and who hold 

 that movements may have arisen without any such bisis, may doubtless be 

 justified in their position. I quite agree that movement mirjlit be developed 

 without circumnutation having anything to do witli the matter. But in seeking 

 the origin of growth-curvatures it is surely rational to look for a widely- 

 spread movement existing in varying degrees. Tbis, as I believe, we have in cir- 

 cumnutation : and liere comes in what seems to me to be characteristic of the 

 evolution of a quality such as movement. In the evolution of structure, each 

 individual represents merely a single one of the units on which selection acts. 

 But an individual which executes a number of movements (which may be purpose- 

 less) supplies in itself the material out cf which various adapted movements 

 may arise. I do not wish to imply that tentative movements are of the same 

 order of importance as variations, but they are undoubtedly of importance as 

 indic^ation of variabilit}'. 



The problem may be talcen back a stnge further ; we may ask why circumnuta- 

 tion should exist. In the ' Power of Movement ' (p. 546) we -wrote : * Why every 

 part of a plant whilst it is growing, and in some cases after growth has ceased, 

 should haA'e its cells rendered more turgescent and its cell-walls more extensile 

 first on one side then on another ... is not Imown. It would appear as if the 

 changes in the cells required periods of rest.' Such periods of comparative rest are 

 fairly harmonious with any theory of growth ; it is quite conceivable by intus- 

 susceptionists and appositionists alike that the two stages of elongation and fixation 

 should go on alternately,' but this would not necessarily lead to circumnutation. It 

 might simply result in a confused struggle of cell.*, in some of which extension, in 

 others elongation, was in the ascendant ; but such a plan would be an awkward 

 arrangement, since each cell would hinder or be hindered by its neighbour. Per- 

 fection of growth could only be attained when groups of contiguous cells agreed 

 to work together in gangs, that is, to pass through similar stages of growth syn- 

 chronou.sly. Then, if the diflferent gangs were in harmony, each cell would have 

 fair play, elongation would proceed equally all round, and the result would be 

 circumnutation.' Whether or no any such origin of circumnutation as is here 

 sketched maybe conceived, there can be no doubt that it had its origin in the laws 

 of growth apart from its possible utilisation as a basis for growth-curvature. 



It is, however, possible to look at it from a somewhat different point of view, 

 namely, in connection with what Vochting has called rectipetality!' He made out 

 the fact that when an organ has been allowed to curve geotropically, helio- 

 tropicall}', &c., and is then removed from further stimulation by being placed on 



' Power of Movemcmt, p. .S. - Loe. cit., p. 4. 



' Strasburger, Histclog. BeHriige, p, 195, speaks of the pause that must occur 

 after the formation of a celhilose lamella. Hofmeister, Wiirttembiirff. Jahrcslieftc, 

 1874, clescribcs the growth in length of Spirogyra as made up of short intervals of 

 rapid growth alternating with long pauses of slow growth. 



* I purposely omit the circumnutation of pulvini. 



' Z>it; Bnvegvng dcr JJliithcn lend Frilchtc, 1882. 



