242 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



SPERMAPHYTA. 



Vegetative organs. In some cases it has been shown that the 

 formation of the primary leaves 1 takes place in feebler light than does 

 that of the succeeding leaves, and that in feeble illumination the plant 

 may be caused to revert to the stage of primary leaves. This is the case 

 in Sagittaria and in Campanula rotundifolia. 



With regard to Sagittaria I have already spoken on page 172. Plants 

 brought into feeble light formed only the strap-like primary leaves. 

 The course of development was otherwise normal as was shown by the 

 fact that the tubers for asexual propagation were formed. There can 

 be no doubt that Sagittaria can be brought back to its primary stage 

 by other influences ; in nature this happens if the plant be in deep or 

 rapidly flowing water. 



Campanula rotundifolia, a typical land-plant 2 , shows like relation- 

 ships. It has two forms of leaves, rounded leaves and linear leaves, which 

 are connected with one another by intermediate forms. The rounded leaves 

 have a kidney-shaped lamina and a long stalk, and are the first leaves 

 produced by the developing seedling at a time when it is growing amongst 

 other plants and is therefore exposed to less intense light than it sub- 

 sequently receives. These rounded leaves are then ' attuned,' so to speak, 

 to light of low intensity. The internodes of the shoot-axis which bears 

 them are short. The linear leaves have no stalks, or these are very short, 

 and the blade is long; they stand upon that portion of the shoot which 

 has elongated internodes and which subsequently in normal conditions ends 

 with flowers. If the plant be cultivated in light of feeble intensity, but yet 

 sufficient for nutrition and for the formation of primary leaves, it can be 

 thrown back to the state in which it forms rounded leaves, even although it 

 has already formed linear leaves (see Fig. 121). I may further remark 

 here that, as my researches have shown, the seedlings .in all circumstances 

 form at first rounded leaves, even in very strong illumination. The spores 

 of Funaria also under these conditions formed a protonema which was not 

 less developed than usual. 



Reproductive organs. That the formation of reproductive organs is 

 linked with the existence of a definite intensity of light follows indirectly 

 from what has been said. In the mosses they arise, as we know, only 

 upon the leafy stem, and in Campanula and Sagittaria formation of 

 flowers is preceded by the ' higher ' form of leaf. 



Sachs was the first who made an accurate investigation of the 

 dependence of formation of flower upon light. A superficial consideration 



1 See the Third Section. 



2 Goebel, liber den Einfluss des Lichtes, &c., ii. Die Abhangigkeit der Blattform von Campanula 

 rotundifolia von der Lichtintensitat, in Flora, Ixxxii (1896), p. I. 



