ASSIMILATION OF CARBON 



27 



closely related to the individual properties of the plant, some forms needing 

 more and other less light. Trees were long ago differentiated by students of 

 forestry into two types, heliophobous (shade plants) and heliophilous (non-shade 

 plants); among the first are included, for example, Abies (fir), Taxus (yew), 

 Fagus (beech), Tilia (linden); among the latter, Pinus (pine), Larix (larch), 

 Betula (birch), Robina (locust). 



Schistostega osmundacea, a moss that grows in dark caves, may be mentioned 

 as an example of plants that can thrive in extremely weak light. Its protonema 

 has a very peculiar structure (Fig. 16), and, although existing in semi-darkness, 

 it appears emerald green. Single filaments of the protonema, as they grow 

 upward, each form a plate of cells lying at right angles to the direction of the 

 impinging light. Each cell of this plate has the form of a lens and the chloro- 

 plasts lie in the prolonged basal region. Acting like biconvex lenses, these 

 cells concentrate the light of the half-dark cave sufficiently to allow carbon 



.4 B 



Pig. 16. — Schistostega osmundacea: A, protonema; B, diagram representing the path taken by 

 rays of light as they enter and leave the cells of the protonema. 



dioxide decomposition by the chloroplasts. A part of the light is reflected, 

 thus rendering the protonema luminous. 



In general, plants are adapted to the minimum of available light (Wiesner, 

 Liubimenko). In heliophilous plants (which thrive best in bright sunshine) 

 the rate of carbon dioxide decomposition increases continuously with increase 

 in light intensity; 5 on the other hand, for heliophobous plants (which thrive 

 in shade or in regions of low light intensity) there exists an optimum light 

 intensity, and any increase beyond this optimum results in a decrease in the 

 amount of carbon dioxide decomposed. This difference is related to the 

 different amounts of chlorophyll contained in the two kinds of plants. Liubi- 

 menko was able to show that heliophobous plants are richer in chlorophyll 

 than are heliophilous ones. Within limits, the greater the amount of light 



« It is not to be understood that there are no optimum light intensities for carbon-dioxide 

 decomposition in plants that grow best in bright sunshine, only that such optima are markedly 

 higher than those for plants that grow best in shade.— Ed. 



