INTRODUCTION 



67 



sometimes cylindric, that is radial. Many leaves also, like those of Iris, 

 are in the main bilateral. 



3. Dorsiventral organs. These, as the name indicates, have a dorsal 

 side and a ventral side which differ one from the other. The two 

 lateral surfaces, the flanks, may be like one another or they may be 

 different. The latter is the case, for example, in the inflorescences of 

 Vicia Cracca which have flowers set in oblique lines along only one side. 



These categories define the most frequent cases only ; that they 

 pass readily into one another has already been shown in the case of 

 Schistostega. Just as an organ laid down as a radial one may become 

 bilateral, so also a bilateral or radial one may become dorsiventral, 

 and many examples of this will be given in the course of the following 

 pages. 



FlG. 25. Schistostega osmundacea. Dis- 

 tichously-leaved plant illustrating bilateral 

 construction. MagniBed. 



FlG. 26. Schistostega osmundacea. Two shoot-apices seen 

 from outside. The primarily transverse insertion of the leaf is 

 displaced towards the long axis of the shoot. 



I must in the next place shortly mention relationships which exist 

 between symmetry and direction of the organ. 



Sachs has divided the organs of plants into the orthotropous and 

 the plagiotropous. An organ is orthotropous if, under usual conditions 

 of life, it grows vertically upwards or downwards when it is illumi- 

 nated equally on all sides ; it is plagiotropous if, under such con- 

 ditions, it assumes an oblique direction to the horizontal plane. The 

 external and internal influences which take a share in this we shall 

 not here consider, but it is important for organography to note that 

 orthotropous organs are almost always radial or bilateral ; plagio- 

 tropous ones, on the other hand, are commonly dorsiventral, seldomer 



F 2 



