INFLUENCE OF EXTERNAL STIMULI. LIGHT 



241 



that is to say, it appears first in the germination in all circumstances 

 and is only dependent in its duration and development in length upon 

 the light, whilst in others its first appearance is determined by light. The 

 question whether the germ-tube represents a phylogenetically older stage 

 does not concern us here. 



MUSCI. In the mosses the phenomena are quite similar. Moss-buds 

 appear upon the protonema only when the intensity of light is higher than 

 that which is required for the normal growth of the protonema l . If 

 the formation of buds does not take place the protonema may theoretically 

 continue its growth to an unlimited extent. As has been previously 

 stated the primordia of moss-buds may, up to a certain stage in their 

 development, be induced to revert to the protonema-form. It is clear 

 that it is of advantage to a moss that the primordia should only develop 

 into moss-plants under conditions which offer to them a prospect of 

 success. The construction of a moss-plant 

 is of a higher character, especially in its 

 capacity for assimilation, than is that of the 

 protonema, and the formation of organs in 

 the leafy moss-plant is, so far as we can 

 judge from insufficient investigations, in a 

 far greater degree dependent upon light 

 than is the case amongst the Pteridophyta 

 and Spermaphyta. 



When etiolation takes place in the 

 Bryophyta the leaf-formation is affected, the 

 unfolding is often retarded, or the leaf is 

 smaller than in plants grown in light ; but 

 in the cases which have been examined the 

 outer differentiation is otherwise unaffected. 

 Fig. 130 represents a leaf of a plant of 



Jungermannia bicuspidata which has been grown in feeble illumination 

 and the normal construction is so greatly simplified thereby that it 

 conforms with the leaves which appear in quite young plants. Similar 

 cases may be found elsewhere. 



FlG. 120. Jungermannia bicuspidata. 

 Portion of a stem grown in feeble light. 

 To the right a leaf which consists of only 

 some two cell-rows, whilst in normal 

 illumination the leaf forms a cell-surface. 



PTERIDOPHYTA. 



Regarding the sexual generation of the ferns we need only mention 

 here that the formation of a cell-surface is associated with a greater 

 light-intensity than is the formation of a germ-filament, and that for the 

 development of sexual organs a high light-intensity is also required. 



1 Klebs, tiber den Einfluss des Lichtes auf die Fortpflanzung der Gewachse, in Biolog. Central- 

 blatt, 1893. 



GOEBEL R 



