TISSUES AND SIMPLE ORGANS. 123 



The most important bearers of the assimilating function are the 

 chlorophyll-bodies, which have a discoid form, among algae a band- 

 like, flattened or stellate structure. A definite tissue in which they 

 may occur is not always necessary, though we usually speak of assim- 

 ilating cells, forming a specific assimilating tissue which is found 

 in the true assimilating organs, the green leaves. The principles 

 which underlie and regulate the function of chlorophyll-bodies, 

 that is, the conditions under which they can perform their most 

 favorable activity, also underlie the structure of the tissues and the 

 organs which serve the function of assimilation. We will there- 

 fore next consider the structure of the assimilating tissue-system. 



(a) The Structural Principles of the Assimilating System. 



G. HABERLANDT and STAHL have within more recent times 

 made important investigations in regard to the physiological anat- 

 omy of assimilation ; to these, among others, must be added the 

 communications of HEINRICHER. From & physiological standpoint 

 the communications of SACHS are the most important. 



The Greatest Possible Utilization of the Luminous Effects of 

 Sunlight by the Chlorophyll. Following in thought the problem 

 suggested by this statement leads us to the essential points of view 

 which give us a physiological understanding of the structure of the 

 assimilating organs. When I say ' ' greatest possible ' ' utilization 

 of sunlight, I wish to explain, in order to avoid erroneous concep- 

 tions, that the nature of chlorophyll is such that assimilation reaches 

 its optimum with certain light-intensities; beyond these an injurious 

 influence makes itself felt. Similarly with the amount of CO 2 

 present : increasing it to 8 per cent with high intensity of light 

 there is still noticeable an increase in assimilation. Physiology 

 must here likewise be satisfied with a causal-final or teleological ex- 

 planation of the anatomical adaptations ; a causal-mechanical ex- 

 planation is impossible. 



The principle of the surface expansion of the leaf which mani- 

 fests itself by the outer form is still more evident in its anatomi- 

 cal structure (Figs. 69, TO, 71). A maximum expansion of cell- 

 surface is obtained by membrane-foldings, by the regular form and 

 large numbers of assimilating cells. Such adaptive arrangements 

 make room for the numerous chlorophyll -grains which are always 

 adjacent to the cell-wall. The regular, elongated palisade-cells. 



