THE PILOBOLUS GUN AND ITS PROJECTILE iii 



of photic equilibrium when the protoplasm lining its inner wall, 

 which is sensitive to photic stimuli, is symmetrically illuminated 

 with the brightest light at its centre. Haberlandt showed that the 

 epidermal cells of the leaves of many plants have a more or less 

 rounded or convex external wall and act as lenses, so that the rays 

 of hght which enter them are refracted and condensed and form a 

 bright spot of light on their inner wall. 



My own ocellar theory of the heliotropic mechanism of the 

 sporangiophore of Pilobolus is essentially similar to Haberlandt's 

 ocellar theory for the heliotropic mechanism of certain leaves. The 

 lens-like subsporangial swelling of Pilobolus corresponds in a leaf 

 to the thousands of lens-like epidermal cells ; and the stipe of 

 Pilobolus, which receives a stimulus from the photically sensitive 

 subsporangial swelling in consequence of which it bends the swelling 

 around until the spot of light within the swelling is symmetrically 

 situated at its base, corresponds in a leaf to the multicellular petiole, 

 which receives a collective stimulus from the thousands of photically 

 sensitive epidermal cells in consequence of which it bends or twists 

 until the thousands of spots of light in the epidermal cells, on the 

 average, have come to be symmetrically situated on their basal walls. 



In the heliotropic reaction of Pilobolus, which is caused by the 

 asymmetrical position of a spot of light in the subsporangial swelling 

 with a consequent bending of the stipe, we have clear proof of 

 the theory, first suggested by Haberlandt, that heliotropic reactions 

 may take place in plants through ocellus action. 



The sporangiophore of Pilobolus is the only ortho-heliotropic 

 plant organ known which takes up its positively heliotropic position 

 owing to the possession of a special light-perceiving cell structure. 



Pilobolus may well be described as a fungus with an optical 

 sense organ or simple eye (ocellus) ; and, in using its eye for laying 

 its gun, it is unique in the plant world. 



The Ocellus of Pilobolus and the Eye-spots of Volvox. — As is 

 well known, red eye-spots are present in many Protozoa, e.g. Euglena, 

 and in the Volvocaceae, e.g. Gonium, Pandorina, PJeodorina, 

 Eudorina, and Volvox ; and it is therefore not without interest to 

 enquire whether or not the ocellus of Pilobolus and the eye-spots 

 of these other organisms work on the same principle. According 



