238 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



new formations on both sides ; if however the illumination be unilateral 

 the new formations arise only upon the illuminated side of the ' leaves.' 

 Similarly 'rhizomes' illuminated from below produce 'leaves' upon the 

 side which previously formed ' roots ' ; dorsiventrality is reversible as in 

 the prothalli of ferns 1 . 



2. QUALITATIVE INFLUENCE OF LIGHT. 



We shall not consider in this place any of the phenomena that are 

 grouped together as etiolation because these are more properly dealt with 

 in physiological textbooks. The following subjects, of importance in 

 organography, are treated of separately here only to enable us to present 

 a general review of them ; it will be understood that they frequently 

 overlap : 



(a) Different developmental stages of one and the same plant are 

 associated with light of different intensity, the earlier develop- 

 mental stages claiming, that is to say being ' attuned ' to, a less 

 intensity of light than the later stages. 



(/>) The flattening and consequent increase of surface of organs 

 containing chlorophyll in consequence of illumination. 



(c] The influence of light in anisophylly. 



(d) The change of function of homologous organs according as 



they grow in light or darkness. 



(c] The influence of light upon the relationships of configuration of 

 Fungi. This will be shortly referred to, facts not having 

 weight in organography being omitted. 



(<?) DEVELOPMENTAL STAGES IN RELATION TO LIGHT. 



When speaking of juvenile forms I have already mentioned examples 

 of what I am now about to refer to here. 







ALGAE. 



The ' pro-embryo ' of Batrachospermum 2 develops the Batrachosper- 

 mum-plant only in bright illumination ; in feeble light the plant remains 

 stationary in the lowest stage of formation of organs, whilst the pro- 

 embryo attains a luxuriant development. The same is the case in other 

 Algae 3 whose germ-plants, if light is feeble, do not develop beyond the 

 stage of the fixing disk or anchoring filaments ; they form no erect 

 thallus, but the fixing disk develops very strongly. 



1 Regarding Eryopsis, see Noll, in Arb. d. hot. Instituts in Wurzburg, iii (iSSS), p. 468. 



2 Sirodot, Les Batrachospermes, Paris, 1884. 



3 Berlhold, Zur Morphologic und Physiologic der Meeresalgen, in Pringsh. Jahrb. xiii. p. 673. 



