The Sense Organs of Plants. 6r 



plumule, while in ordinary leaves it is often the leaf-blade. The 

 leaf-blade, when stimulated by light, transmits a signal to a remote 

 part, often the leaf-stalk. If the leaf-blade is not already in the 

 most favourable position, the stalk bends or twists in a suitable 

 direction, and will even in many cases remove the blade out of 

 the shade caused by (Uhcr leaves. Thus again, as in organs 

 sensitive to gravity, we can distinguish a perceiving and a motor 

 region. 



If the perceiving region, the leaf-blade, is examined an- 

 atomically, it is found that in many cases the cells of the upper 

 epidermis are shaped like convex or planoconvex lenses. Being 

 filled with a clear cell-sap, these ceils actually behave like lenses, 

 and cause a convergence of the light rays, which are brought to a 

 focus somewhere inside the leaf. In other plants special cells, or 

 local thickenings of the cuticle act in the same way. 



It seems highly probable that these leaf lenses, together with 

 the surrounding cells, constitute the light perceiving organs of the 

 leaf. They are comparable, perhaps, to the compound eye of an 

 insect. There is some difference of opinion as to the exact 

 modus operandi of these sense organs. Probably they bring about 

 a stimulation of the leaf in some way by unequal illumination of 

 different portions of the living protoplasm. It is possible that 

 certain portions are, so to speak, attuned or accustomed to light 

 of a certain relative intensity. Any departure from the normal 

 conditions of illumination, such as that caused by an alteration in 

 the position of the leaf, would then be attended by excitation of 

 the protoplasm, and subsequent readjustment of position. 



As a proof that the epidermal cells do actually behave as 

 lenses, I will throw on the screen some beautiful slides from 

 photographs by Mr. Harold Wager. These photographs were 

 -taken under the microscope, using pieces of stripped off epidermis. 

 They show that the lens-cells can actually focus an image of 

 external objects in the interior of a leaf, just as the lens of a human 

 eye can form an image on the retina. There the analogy must 

 stop, for there is no particle of evidence that the plant is in any 

 way conscious of the images so produced in the cells of its leaves. 



