62 



THE FACTORS 



[Part I 



intensity, beyond which light exerted a retarding effect. In the case of 

 shade-plants, the optimum is attained under a much weaker illumination. 



Thus the leaf of Scolopendrium officinarum attained in the dark a length of 76 mm. 

 (breadth 11 mm.), when the full light intensity was 0-083 the maximum length of 

 228 mm. (breadth 25 mm.) was reached, and when the full light intensity was 0-247 it 

 had a length of only 152 mm. (breadth 20 mm.). On the other hand, positive illumina- 

 tion always has a retarding effect on the growth of the stem. Potato-shoots show 

 this reaction plainly even when the full light intensity is 0-0008, whereas diminished 

 leaf-growth is not apparent until the full light intensity is 0-451. 



All these values are however in absolutely saturated air, but the effects of light 

 are by no means eliminated in consequence. 



The minimum of light for the formation of flowers is lower for shade- 

 plants than for sun-plants ; yet the former generally produce fewer flowers 

 than do the latter. The interior of a forest is poorer in flowers than 

 a meadow, and certain regions with intense or prolonged illumination, such 

 as the higher regions of vegetation in mountains, polar countries and many 



deserts, are characterized 

 by a great abundance of 

 flowers. In such cases, 

 however, other factors co- 

 operate. 



Apart from their ex- 

 ternal form, sun-plants 

 and shade-plants respec- 

 tively differ from one 

 another in their internal 

 structure, and especially in 

 that of their foliage. The 

 formation of palisade-cells 

 is favoured by intense light, just as it is by droughts. Leaves exposed to the 

 sun contain chlorophyll in their mesophyll only, but shaded leaves, over and 

 above this, contain chlorophyll, sometimes even chiefly, in the epidermis. 



Of special interest in many plants living in the shade are the contrivances 

 for concentrating the rays of light on the chloropJiyll-apparatns. The 

 existence of an illuminating mechanism in plants was first demonstrated 

 and its nature carefully studied by Noll, in the case of the protonema of 

 Schistotega osmundacea, the luminous moss that lives in caves (Figs. 39, 

 40). This protonema, which alone possesses the property of emitting light, 

 is tabular in form and consists of a slender foot bearing a flat two-lobed 

 plate. The cells of the latter are lenticular and spheroidal above, but 

 produced conically below ; the chlorophyll-corpuscles are aggregated in the 

 narrow basal part of the cells, the upper part of which acts as a completely 

 hyaline glass-like lens. As Noll shows in detail, the incident rays that are 



Fig. 39. Schistotega osmundacea. Protonema in its natural 

 position. Highly magnified. After F. Noll. 



