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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



the incident rays of light is perceived mainly by the leaf-blade, and 

 that this stimulus is transmitted to the petiole, by which the turning 

 of the leaf is brought about. The recent researches of Haberlandt 

 have awakened fresh interest in this question. That author endeavoured 

 to explain by what means the leaf perceived the most advantageous 

 position for the incident rays. He showed that in many plants the 

 upper epidermal cells are shaped like convex lenses, and being filled 

 with a clear sap are able to bring about a convergence of light rays ; in 

 others, special cells or local thickenings of the cuticle act in the same 

 way. Haberlandt suggested that these cells are functional as ocelli, or 

 primitive eyes, capable of setting up a stimulus which results in the 

 heliotropic orientation of the leaf. The epidermal cells have a thin 

 layer of protoplasm on their basal walls. When the leaf is at right 

 angles to the light, the central portion of this layer in each cell is 

 illuminated, the peripheral zone remaining dark. In oblique illumina- 

 tion the bright spot of light moves to one side ; and this alteration in the 

 position of the light spot, according to Haberlandt, sets up the stimulus 

 which results in the orientation of the leaf into a more favourable 

 position. The evidence for this conclusion is based upon the optical 

 behaviour of the epidermal cells and upon experiments by which the lens 

 function is eliminated. 



In the present paper the author criticizes some of Haberlandt 's 

 conclusions and gives the results of his own researches on the subject. 

 He notes first of all the efficiency of the lens cells in question by pointing 

 out that in many leaves they are able to form clear images of objects 

 focussed through them, which may be easily seen under the microscope 

 and can be photographed. The following is a slightly abridged account 

 of the author's own summary : 



Haberlandt 's hypothesis is open to criticism both on morphological 

 and on physiological grounds. The phenomenon of convergence of light 

 by the cells of plants is of very widespread occurrence. Not only the 

 epidermal cells of leaves, but all cells which through turgidity assume a 

 spherical or cylindrical form are capable of bringing it about. The cells 

 of the lower as well as the upper epidermis are in most cases capable of 

 light convergence. Special lens cells and lens- shaped thickening of 

 the cuticle often occur on the lower epidermis. The position of the 

 lens-shaped thickening of the cuticle in Garry a elliptica, which occurs 

 on both sides of the leaf, has no relation to the position of the epidermal 

 cells. 



According to the form and outline of the cell the rays of light may 

 be converged to a local point, a focal line, or to an irregular figure inter- 

 mediate between these. Cells with very irregular outlines, such as 

 Eranthis hyemalis, commonly have more than one series of converging 

 rays, each producing a bright spot of light. Cylindrical cells which 

 bring rays to a focal line are present on some orthotropic organs, such 

 as stalks of leaves, pedicels of flowers, and hypocotyls of seedlings. 

 Papillate cells and lens-shaped thickenings are found on leaves which 

 are not heliotropically sensitive. 



