68 



RELATIONSHIPS OF SYMMETRY 



bilateral as in the bilateral shoots of Schistosteo;a and Fissidens. 



o 



Familiar illustrations of this are the orthotropous chief axes of herba- 

 ceous plants of which the radial configuration is visible without 

 further investigation, and the ordinary plagiotropous leaves in which 

 dorsiventral structure shows itself in the differences between upper and 

 under surface, whilst the radially constructed leaves of Juncus, which are 

 commonly, and indeed in part perversely, called ' sterile haulms,' are 

 orthotropous just as are the bilateral leaves of Iris and other plants. 



The seldom occurrence of bilaterality 

 in plagiotropous organs 1 stands in con- 

 nexion with the fact that most of them, 

 especially under the influence of one- 

 sided illumination, have become dorsi- 

 ventral. We consider that in most cases 

 the plagiotropous direction, caused by 

 external and internal factors, is the 

 primary one, the dorsiventral construc- 

 tion is secondary. We may also say 

 that the radial character of most sub- 

 terranean plagiotropous organs is con- 

 nected with the fact that light has a 

 special significance in the determina- 

 tion of dorsiventrality. Once an organ 

 has imprinted upon it a dorsiventral 

 character it reacts towards outer influ- 

 ences differently from a radial one. 



The relationships vary very much 

 in details. One and the same organ 

 may in different stages of its develop- 

 ment be orthotropous and then plagio- 

 tropous, or, in consequence of the 



different influences of external stimuli, 



FIG. 27. Hypnum (Hyiocomium) spiendens. it may behave either as an orthotropous 



Tiered growth. The shoot of each year at first J 



orthotropous becomes plagiotropous and branches or a plaPiotrODOUS Structure. Ol", beCaUSC 



in one plane. Natural size. 



of relationships of correlation especially, 



plagiotropous organs may pass over into the orthotropous condition ; 

 we find also not infrequently that the vegetative shoots are plagiotropous 

 whilst those of propagation are orthotropous 2 ; this occurs, for example, 



1 See the examples cited above. 



2 I have before now referred to the fact that the shoots of many dicotyledonous plants (Gentiana 

 asclepiadea, species of Lonicera), which in free illumination on every side are orthotropous, become 

 plagiotropous in restricted unilateral illumination because this gives them a better means of utilizing 

 the light. See Beitr. zur Morphologic tind Physiologic des Blattes, in Botan. Zeitung, 1880, p. 753. 



