DORSIVENTRAL SHOOTS. ANISOPHYLLY IN PTERIDOPHYTA 103 



system and in their cortex 1 , whilst the orthotropous shoots of Lycopodium 

 Selago are radially constructed in these respects. Anisophylly appears 

 on the other hand in the cycle of affinity of Lycopodium complanatum 

 (L. complanatum, L. alpinum). In Lycopodium complanatum the plant 

 possesses a subterranean chief axis from which lateral shoots of limited 

 growth pass out into the light. It is not superfluous to remark regarding 

 these Pteridophyta that the subter- 

 ranean parts have no chlorophyll. The 

 aerial shoots, especially the branches of 

 higher order, are strongly flattened, 

 markedly dorsiventral and anisophyl- 

 lous (Fig. 55). There are four rows of 

 leaves, an upper, two lateral, and one on 

 the under side ; on stronger branches 

 there are more. The lateral leaves, 

 which along with their leaf-cushions 

 (the portions of the leaf-bases ' concres- 

 cent' with the shoot-axis) do almost all 

 the work of assimilation, are not only 

 larger than the upper and under leaves, 

 but have also a different form ; they 

 are flat in their apical part like the 

 leaves of the two other rows but in their basal part they are keel-like 

 (Fig. 56, i), in other words their configuration shows an approach 

 to the form of leaves met with in Fissidens. The keel which is con- 



FlG. 55. Lycopodium complanatum. Dorsiven- 

 tral shoot. The figure on the left shows the side 

 turned to the light ; that upon the right shows the 

 shaded side. Magnified about ii. 



FIG. 56. Lycopodium complanatum. Transverse section through a dorsiventral shoot of higher order; I at the 

 apex ; 2 lower down. S shaded side, L illuminated side. 



tinned downwards into the leaf-cushion is much richer in chlorophyll 

 upon its upper side than upon its under side, and the whole configuration 



1 See Hegelmaier in Botan. Zeitung, 1872, p. 776 : The cortex is developed upon the illuminated 

 side into convex ribs descending from the insertions of the leaves, that is to say, leaf-cushions are 

 present, whilst on the shaded side these ribs are wanting, that is to say, there are no leaf-cushions. 

 The strongly dorsiventral shoots of Lycopodium complanatum show the same features in a greater 

 degree (see the text). The species of Lycopodium furnish instructive illustrations of the different 

 degrees in which allied forms may react to external stimuli which influence configuration ; what 

 I relate in the text of my investigation of the case of Lycopodium complanatum affords excellent 

 proof of this reaction. 



