290 PHOTOSYNTHETIC SYSTEM 



the mesophyll is well illustrated by the fertile region of the leaf of 

 Asplenium Brlii a <jrri, where nothing but a single stratum of funnel-cells 

 and one layer of spongy parenchyma intervenes between the upper and 

 the lower epidermis (Fig. 116 e). 



The above-mentioned funnel-cells require a little more notice. In 

 many shade-plants they take the place of ordinary palisade-cells even 

 on the upper side of the leaf. In some cases e.g. Oxalis Acetosella 

 the chlorophyll-corpuscles are confined to the lateral walls in these 

 funnel-cells ; here the advantage of this particular shape of cell seems 

 to consist in the fact that the chloroplasts lie, as it were, half-way 

 between the profile- and surface-positions, and thus receive more 

 light than they would if they were permanently fixed in the profile- 

 position, which is an unfavourable one where the average illumination 

 is feeble. Noll has shown that chloroplasts obtain still better illumina- 

 tion, if they are located at the base of funnel-shaped cells, a condition 

 which is exemplified by certain species of Selaginella (Fig. 97). 

 Here the funnel-like shape causes the cells to act as condensing lenses. 

 Owing to the concave form of the outer wall, namely, the rays of 

 light that fall perpendicularly upon the leaf-surface are made to con- 

 verge, by refraction and total reflection, towards the base of the cell, 

 so that the chloroplasts assembled at that end of the cell are more 

 brightly illuminated than they otherwise would be. The same optical 

 device is found in a much more perfect form in the protonema of 

 Schistostegcc osmundacea. Here the chlorophyll-apparatus in each 

 funnel-shaped cell is so brilliantly illuminated, that it acts against a 

 dark background as a self-luminous body, any rays that are not 

 absorbed by the chloroplasts being reflected and retraversing the 

 whole optical system in the sense contrary to their path of entry. 

 We are indebted to Noll u7 for the above explanation of the well- 

 known luminosity of this protonema. 



The deviation of the funnel -cell from the normal palisade-form 

 is obviously connected with special conditions of illumination : but 

 there are modifications in respect of shape and orientation of another 

 sort which are related to the function of translocation, and yet others 

 that result from certain developmental processes, and the physio- 

 logical value of which is obscure if it exists at all. Where the 

 palisade-tissue is loosely put together, individual cells thereof are 

 frequently found to assume an oblique position or to bend to one side 

 in order to reach a collecting-cell which is situated some distance away. 

 Similar displacements often take place in the case of palisade-cells that 

 underlie water-absorbing or water-secreting trichomes. Sometimes all 

 the palisade-cells of a leaf (or green stem) are obliquely directed, a 

 circumstance first discovered by Pick ; this peculiarity probably does 



