DORSIVENTRAL SHOOTS. ANISOPHYLLY IN PTERIDOPHYTA 105 



but also and particularly the manner in which the plant on account of 

 its material nature responds to this. 



The different species of Lycopodium furnish also an illustration of 

 the varying sensitiveness to the influence of external factors existing in 

 one genus. The plagiotropous shoots of Lycopodium annotinum and 

 others are only feebly dorsiventral in respect of the development of their 

 cortex and vascular system. In Lycopodium alpinum the dorsiventrality 

 increases and approaches nearly to that of Lycopodium complanatum ; 

 the leaf-cushions upon the illuminated side are in this species more 

 developed than they are upon the shaded side (see Fig. 57), and the 



FlG. 57. Lycopodium alpinum. A scries ot" transverse sections through a shoot at different heights, i being the 

 highest, 4 lieing the lowest. /, the illuminated side is turned upwards, .V the shaded side is turned downwards. In 

 2, 3, and 4, the portion of the leaf-cushion which is richest in chlorophyll is indicated by shading. 



leaf-cushions of the lateral leaves are strongly flattened and dorsiventral ; 

 but the lateral leaves themselves are not so altered as they are in 

 Lycopodium complanatum. They are usually flat and experience a kind 

 of torsion only in their basal portion where it passes into the leaf- 

 cushion, and here they also become feebly keel-like (see Fig. 57) ; 

 the leaves on the shaded side are more strongly developed than in 

 Lycopodium complanatum. The leaves which are found upon the under 

 side of the feebly dorsiventral creeping shoots of Lycopodium inundatum 

 are also smaller than the others and often have no chlorophyll. 



Selaginella. In the extensive genus Selaginella 1 we find both 

 isophyllous and anisophyllous species. The latter, which are the most 

 numerous, possess four rows of leaves, and the leaves of the two rows 

 which are turned to the light are smaller than those on the under side 

 of the stem, but there is one species which according to external circum- 

 stances may be either isophyllous or anisophyllous. This is Selaginella 

 sanguinolenta which I have carefully investigated, led thereto by a remark 

 of Spring in his Monograph of the Lycopodiaceae. 



This species which usually lives on stony spots 2 and, as we may 

 conclude from its anatomy, in places which are, at least sometimes, dry. 

 possesses erect shoots with four rows of adpressed thick leaves which are 



1 Baker, Handbook of the Fern-allies, enumerates 334 species of which eight are isophyllous. 



2 The isophyllous species of Selaginella usually grow on dryer and brighter spots than the 

 anisophyllous. I at least come to this conclusion from my observation of S. spinulosa and S. rupes- 

 tris ; the adaptation to periodical drying up of species like S. lepidophylla amongst anisophyllous 

 forms is a secondary phenomenon. 



