560 ZONES AND REGIONS [Pt. Ill, Sect. II 



season many perennial herbs pass through the whole cycle of their de- 

 velopment above ground, and after the canopy of leaves overhead is 

 completed they continue to live only in their subterranean parts, as we 

 see in Central Europe in plants like Anemone, Adoxa, Corydalis. Decided 

 shade-plants on the other hand remain green according to the degree of 

 their ability to assimilate even when the light is weak, for, after the com- 

 pletion of the leaf-canopy, assimilation remains their sole important function 

 dependent upon light. Such is the behaviour, for instance, of ferns, Oxalis 

 Acetosella, and like plants in Central Europe. Hemisaprophytes are 

 among the most accommodating green plants as regards illumination, and 

 flower even at mid-summer, for instance, Pyrola, Goodyera, Listera cordata, 

 Corallorhiza. Holosaprophytes, such as Monotropeae, Epipogum, numerous 

 fungi, as well as the majority of parasites of the forest, such as Lathraea 

 and Clandestina, occur in the deepest shade. 



Unfavourable as are the conditions in regard to light in the shade of the 

 forest, yet they represent the optimum for most of its characteristic plants, 

 and especially for its terrestrial herbs. Still one often finds in the deepest 

 shade a thin covering of grasses and of other stunted little plants which 

 never flower, and by this as well as by their whole appearance show that 

 more light is necessary for their development ; and the few individuals 

 which develop from the seeds of the trees forming the canopy remain 

 dwarfed until an opening in the canopy permits the entry of more light. 

 But the plants that we notice, particularly in spring, healthy despite their 

 delicacy, blossoming and fruiting, find themselves in a situation where their 

 optimal conditions reign. They are, however, confined to places where 

 these conditions occur, and therefore avoid the deepest shade. Wiesner, 

 whom we have to thank for valuable investigations regarding vegetation 

 in the shade, has shown that the photic ration of Hepatica triloba in its 

 natural habitat corresponds to the most suitable degree of illumination. 

 An increase or a diminution of this optimum intensity of light alike call 

 forth a reduction in the leaves that is never displayed in the natural 

 habitat. 



At the time when Hepatica triloba develops its leaves and flowers, the beech- 

 forest is still leafless and the soil is therefore comparatively well illuminated ; 

 the plant blossoms with L jL to £, usually with i-J (I max. = 0-499-0-333) ; 

 I med. = 0-242-0-166. During the somewhat later (middle of April) leaf-develop- 

 ment, the forest is as yet only slightly in leaf; the intensity of light under the shade 

 of the beech-trees at this time is about J-£ (I max. = 0-123-0-329 ; I med. = 0-062- 

 0-171). One finds the full-grown leaves, normal and functioning down to L y 1 - and 

 below ; in the extremest case, however, they no longer assimilate and they 

 languish at L 2V (I max. = 0-036). 



The following data are taken from Wiesner's observations regarding the influence 

 of illumination in the development of the Hepatica: — 



