6a 



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 1 52 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 eflects of light 

 are by no means eliminated in consequence. 



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

 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 manyj 



deserts, are characterized 

 by a great abundance oi 

 flowers. In such cases 

 however, other factors co- 

 operate. 



Apart from their ex- 

 ternal form, sun-plants 

 and shade-plants respec- 

 ti\"cly differ from one 

 another in their interna! 

 structure, and especially in 

 that of their foliage. The 

 formation of palisade-cell.' 

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

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

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



Of special interest in many plants living in the shade are the contrivance. 

 for concentrating the rays of light on the chlorophyll-apparatus. Tht 

 existence of an illuminating mechanism in plants was first deinonstratec 

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

 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-lobec 

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

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

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

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



Fig. 39. Schistotega osmundacea. Protonem.-i in its natural 

 position. Highly magnified. After F. Noll. 



