OPENING AND CLOSING OF FLOWERS. 6^t 



all that has been said of the proper motile organs, applies to these cases also, at 

 least according to PfefFer's account, only with the difference that the important 

 matter here is not, as there, a periodic lengthening and shortening of the upper 

 or lower side of the organ, but each elongation with its consequent curvature is. 

 permanent ; when the other side becomes elongated, with a corresponding curvature' 

 in the opposite direction, this elongation is also permanent, and so on-^i. e. the 

 upward and downward curvatures are here caused by first the one and then the other 

 side growing more vigorously. 



Fundamentally, however, there is no difference whatever in principle, as' 

 is evident from the fact that even in the case of the leaves of the group first 

 considered the periodic movements commence before the motile organs are fully 

 grown. Still more important, however, is the following consideration ; it has' 

 already been shown in previous lectures that growth in general depends upon 

 the turgescence of the cells, and that as the turgescence of the tissue increases, 

 its growth is accelerated, and as it diminishes, it is retarded. If now changes in' 

 turgescence are caused by illumination and darkening, just as in the motile organs 

 of the first group, so also changes in the rapidity of growth must occur in growing 

 organs. In this case, for upward and downward curvatures to result, the altera 

 ations in turgescence here also must take place more quickly on the one side of the. 

 organ in question than on the other. ' 



From PfefFer's descriptions it is particularly clear in the case of the young leaves 

 of Impatiens noli-me-iangere, which are very sensitive to variations of light, that they: 

 agree in every respect, apart from the difference just explained, with the leaves 

 of the group first considered above. 



Pfeffer established in 1873 that those corollas which exhibit so-called sleep- 

 movements — i. e. which open and close at certain hours of the day — likewise owe 

 this periodic movement to periodic alterations in the growth in length of the outer 

 and inner sides of the petals. Since, therefore, we are here no longer con-: 

 cerned with a new principle, but only with peculiarities of floral structure, and 

 specific differences, I shall desist from descriptions in detail, and only dwell on 

 one point. 



■While in the case of the foliage-leaves of the first group, the effect of changes 

 of temperature is of entirely subordinate importance, compared with the paratonic- 

 action of variations of light, in the case of the foliage-leaves of the second group, 

 where growth is the prominent feature, the variations of temperature exert 

 great infliience. But it is in periodically motile flowers that sudden variations 

 of temperature induce the most active movements, and of such a kind that 

 an elevation of temperature causes the flowers to open, and thus produces an 

 outward curvature of the petals, while sudden cooling effects their closure,' 

 that is an inward curvature, by the more active growth of the outer side. 

 In addition to the flowers of the Crocus and Tulip,. Pfeffer mentions as very, 

 sensitive to changes in temperature the corollas of Adonis vernalis, Ornithogalurw 

 umhellalum, Colchicum auiumnale — i.e. particularly flowers which are apt to open 

 in the Spring or late Autumn, when the temperature of the air is low, and 

 which are only occasionally warmed by the sun. Less sensitive are those of 

 Ficaria ranunculoides, Anemone nemorosa, and Malope Irifida, all of which execute 

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