256 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



The configuration of underground organs removed from the light 

 differs usually considerably from that of homologous organs which are 

 exposed to the light l . Thus rhizomes and stolons have scale-like 

 cataphylls instead of foliage-leaves. The difference does not always 

 depend directly upon light ; but this is the case in Circaea 2 . Its seedling 

 plant at first forms only one aerial shoot ; in the axil of the lowermost 

 foliage-leaves shoots arise which bend towards the earth, and upon these 

 are developed at first foliage-leaves, which apparently in conformity with 

 the inclined growth of the axis are smaller than those of the orthotropous 

 shoot. As soon as the shoots enter the soil, they form scale-leaves instead 

 of the foliage-leaves. But if one of these stolons be allowed to grow 

 permanently with access to light, for example, in an illuminated water- 

 culture, it forms instead of the scale-leaves small foliage-leaves, which 

 do not lie close upon the axis, like the scale-leaves, but stand out from it, 

 and thus exhibit a character which is associated with the possession of 

 chlorophyll 3 . 



The stolons of the Hieracia behave in like manner, whilst those of 

 Adoxa moschatellina form scale-leaves even under illumination. 



The Algae especially furnish similar examples. Oedocladium proto- 

 nema 4 is an alga living on land and having aerial much-branched green 

 threads and subterranean thin sparingly-branched colourless ' roots.' If 

 the latter be exposed to the light, they become green and are transformed 

 into normal short-membered aerial twigs. On the other hand, on plants 

 from which the 'roots' have been removed, new 'roots' arise in a short 

 time, which are distinguished by their sparingly green colour and their 

 negative heliotropism. Sometimes these are new formations, sometimes 

 they proceed out of the apex of green twigs, from which I conjecture that 

 relationships to light are critical. Berthold 5 has observed the following 

 in marine Algae : ' If one brings specimens of different filamentous Algae, 

 for example, Callithamnion, Bryopsis, Ectocarpus, from the sea into a 

 culture-vessel in feeble illumination, the apices immediately grow out into 

 rhizoid-like threads ' ; and Noll 6 has been able, in specimens of Bryopsis 

 planted in an inverted position, to cause the short green twigs, even the 

 apex of the plant itself, to develop into rhizoids, which are otherwise only 

 formed at the basal end of the plant Here however the condition of the 



1 This of course is entirely apart from such differences as absence of chlorophyll. 



2 Goebel, Beitr. zur Morphologic und Physiologic des Blattes, in Botan. Zeitung, 1880, p. 794. 



3 Orthotropous shoots of Circaea do not produce scale-leaves in the dark, but etiolated foliage- 

 leaves, and the shoots referred to in the text have evidently a definite peculiarity which determines 

 their reaction in the way described. 



* E. Stahl, Oedocladium protonema, eine neue Oedogoniaceengattung, in Pringsh. Jahrb. xxiii. 



5 Berthold, Morphologic und Physiologic cler Meeresalgen, in Pringsh. Jahrb. xiii. p. 673. 



6 Noll, Uber den Einfluss der Lage auf die morphologische Ausbildung einiger Siphoneen, in Arb. 

 d. bot. Instituts in \Yurzburg, iii. p. 466. 



