8 president's address. 



Morphological Changes, 



Let us pass on to the consideration of the permanent or morphological 

 changes and the stimuli by which they are produced, a subject to which, 

 in recent years, many workers have devoted themselves. I need only 

 mention the names of Vochting, Goebe.l, and Klebs among botanists, and 

 those of Loeb, Herbst, and Driesch among zoologists, to remind you of 

 the type of research to which I refer. 



These morphological alterations produced by changes in environment 

 have been brought under the rubric of reaction to stimulation, and must 

 be considered as essentially similar to the class of temporary movements 

 of which I have spoken. 



The very first stage in development may be determined by a purely 

 external stimulus. Thus the position of the first cell-wall in the develop- 

 ing spore of Equisetum is determined by the direction of incident light." 

 In the same way the direction of light settles the plane of symmetry of 

 Jfarchantia as it develops from the gemma.- But the more interesting 

 cases are those where the presence or absence of a stimulus makes an 

 elaborate structural diflference in the organism. Thus, as Stahl ^ has shown, 

 beech leaves developed in the deep shade of the middle of the tree are so 

 different in structure from leaves grown in full sunlight that they would 

 unhesitatingly be described as belonging to different species. Another 

 well-known case is the development of the scale-leaves on the rhizome of 

 Circma into the foliage leaves under the action of light.'' 



The power which the experimenter has over the lower plants is shown 

 by Klebs, who kept Saprolegnia mixta, a fungus found on dead flies, in 

 uninterrupted vegetative growth for six years ; while by I'emoving a 

 fragment of the plant and cultivating it in other conditions the repro- 

 ductive oi'gans could at any time be made to appear.'^ 



Chlamydomoyias media, a unicellular green alga, when grown in a 

 04 per cent, nutrient solution continues to increase by simple division, 

 but conjugating gametes are formed in a few days if the plant is placed 

 in pure water and kept in bright light." Numberless other cases could 

 be given of the regulation of form in the lower organisms. Thus Sporo- 

 dinia grown on peptone-gelatine produces sporangiferous hypha, but on 

 sugar zygotes are formed. Again, Protosiphon botryoides, if grown on 

 damp clay, can most readily be made to produce spores by tranference to 

 water either in light or in darkness. But for the same plant cultivated in 

 Knop's solution the end can best be obtained by placing the culture in the 

 dark.^ Still these instances of the regulation of reproduction are not 

 so interesting from our point of view as some of Klebs' later results/ 

 Thus he has shown that the colour of the flower of Campanida trachelium 



' Stahl, Ber. d. Bot. Ges., 1885, p. 334. - Pfeffer, in Sachs' Arbeiten, i. p. 92. 



» JenaiscJw Zeitsch:, 1883, p. 1(;2. ■• Goebel in Bot. Zeitung, 1880. 



■'' WiUMrUclie Entmclt., p. 27. '■ Klebs, Bcdingvngen, 1896, p. 130. 



• Binl. Cfvtrnm. 1904, pp. 451-3. * JaJirh./. iviss, Bot,, xlii. 1906, p. 168. 



