100 



PLANT SOCIOLOGY 



oerniination increases very rapidly with increase of light at extremely 

 low intensities, then more slowly, so that this light curve of germination 

 has the logarithmoid form of the assimilation curve. The law of 

 relativity evidently applies here also (Fig. 54). 



With a Hght intensity of 200-m. candles for 5 min. 43 per cent of 

 the seeds germinate, a result which follows only with a 3-hr. exposure 

 to a light of 5-m. candles. 



Position of Shoots in Relation to Light.— In intense light leaves 

 and shoots are usually placed parallel to the incident rays (compass 

 plants). In weak light the leaves are placed transversely to the rays. 



7 <y ^ 10 11 1Z 1i Tf- 15 



M/nufes of exposure 



Fig. 54. — The influence of light exposure on the germination of the seeds of Lythrum 

 salicaria. MC = Meter candles. {After Lehmann and Lakshmana.) 



Long-continued intense illumination induces a narrowing of the 

 stomatal pores (Burgerstein, 1919). 



As shown by laboratory experiments and by direct observation 

 in deep shade and in caves, reduction of hght is followed by elongation 

 of shoots, enlargement of laminae of leaves, and change in shape of 

 leaves and stems (Vochting, 1894). Hand in hand with changes in 

 outer form go changes in structure of the tissues. Leaves from shade 

 or darkness show a separation of the mesophyll cells and often com- 

 plete disappearance of palisade tissue. The number of layers of cells is 

 less than in sun leaves, and the air spaces between the cells are larger 

 (Fig. 55). 



The morphogenic action of light upon the structure and develop- 

 ment of leaves and roots has been studied by Schantz (1918), Maximov 

 and Lebendinceff (1923), etc. Schantz found that leaf structure is 

 greatly modified by shading from the short-wave ultra-violet light. 

 The more the light of short wave length is withheld from the plant the 



