SUPPLEMENT in 



problem must be elucidated. As we have previously seen, KLEBS assumes that 

 the external factors always effect certain internal changes which in their turn 

 operate on the specific structure of the plant. KLEBS attempted earlier (1903, 

 p. 67) to prove that different external factors which induce the same morpho- 

 genic results affect the internal conditions in the same way. Thus, for example, 

 a reduction of osmotic pressure in Vaucheria should in general be followed by 

 the formation of swarmspores. More recently, however (1904, p. 492), KLEBS 

 rightly discards this idea and admits that the first internal changes in the cases 

 1-6 may be very distinct. At the same time we must confess that we know 

 absolutely nothing as to the mode of operation of external factors. To us it does 

 not appear to be at all certain that these agents must always, in the first instance, 

 influence internal conditions ; we cannot see why it should not be possible that 

 the influence of the determinating part of the protoplasm in such simple cells 

 living in water should be direct. 



We suggested in the first edition of this book that external factors operate 

 as releasing stimuli. This view was based on the comparison of an organism 

 and a machine. What KLEBS has put forward against this comparison (1904, 

 1905) appears to us to be both correct and worth noting. The comparison is, 

 like all comparisons, imperfect, and perhaps very much so ; still, we must con- 

 tinue to employ it, if we would seek to acquire any essential knowledge of the 

 effect of external factors on plant organogenesis. Let us assume that we are 

 dealing with a machine with threefold powers, e. g. a machine that can stitch, 

 embroider, and knit, and that it is driven by steam. The fly-wheel set in motion 

 by the steam can render one or other of these powers operative by bringing into 

 or throwing out of gear a certain small wheel, and this latter action may be 

 brought about by external forces such as heat, light, gravity, chemical forces, &c. 

 The external forces do not actually perform the work, but merely permit the 

 manifestation of this or that activity. Similarly, in the organism certain 

 external factors operate as releasing agents ; they subtract or add something, 

 and in consequence the activity of the protoplasm responds in a definite way. 

 This is the idea we have in mind though we would willingly replace it with 

 a better if we had one. 



Although studies hitherto undertaken on these fundamental problems 

 have not led us to any explanation of them, still we must not underestimate 

 their results ; the knowledge of the fact that the developmental cycle in lower 

 plants is determined by the environment is of importance. 



358, 1. 30, after entirely different, read By altering external conditions 

 we can, at least in many plants, fundamentally alter the typical life-cycle. 

 Changes of this kind have been effected by KLEBS on Sempervivum Funkii. 



359, 1. 3, for 1898-1901, p. 430 read Organography , p. 607). 



362, 1. 9, after these read buds 



I. 28, for water read materials 



363, 1. 42. after (1893) read and KLEBS (1903-1906) ; for has read have 



II. 43-4. for The plant . . . which left off read One may either start from 

 the vegetative stage and determine what changes in the environment lead to 

 flower formation or, conversely, under what conditions a plant at the flower- 

 ing stage may be induced to return to the vegetative stage. Hitherto no 

 conclusions of general applicability have been established, for a factor which 

 leads to one result in one plant is often in another without any effect. 



Taking Sempervivum Funkii as an illustration (a plant already referred to), 



KLEBS (1906, p. 116) has summarized the results of his experiments as follows : 



i. When carbon assimilation is active in bright light, and when water and 



