HUTCHINSON'S POPULAR BOTANY 



palisade-parenchyma. These palisade cells vary in length in different species. 

 Professor Haberlandt suggested that in certain plants the epidermal cells 

 act as ocelli, or primitive eyes, to the plant. The structure of these cells is 

 often lens-shaped, and consequently the rays of light which fall upon them 

 are brought to a focus. He found that by using such cells as lenses he 

 could obtain minute photographs of various objects, the image being 

 focussed upon the basal wall of the cell. When this fact became known in 

 England a few years ago, some of the more sensational newspapers made 

 capital out of it, and explained how plants could see, like animals, and they 

 published drawings that were supposed to be photographs of things " seen 

 through the eyes of plants." Of course, the plant has no nervous mechanism 

 that will enable it to see. For a human being to separate one of these cells 



and use it as a lens by which to 

 obtain on the human retina a 

 diminished image of some object 

 is one thing ; for plants to be 

 able to see with that lens is 

 quite another matter. But ac- 

 cording to Haberlandt's inter- 

 esting hypothesis, the converg- 

 ence of the light-ra}^ that pass 

 into these lens-shaped cells 

 causes a differential illumination 

 of the protoplasm on the basal 

 walls of these cells, and sets up 

 a stimulus which results in the 

 leaf being moved into that 

 attitude in which it can ob- 

 tain the most suitable illumi- 

 nation for its work, in which 

 light plays so important a 

 part. Not only is this function being performed by the cells on the 

 upper surface of the leaf, but in a modified degree by those on the 

 lower surface. 



It is believed that this convergence or focussing of the light results in 

 the more efficient illumination of the chlorophyll grains. Mr. Harold Wager, 

 F.B.S., who has made many experiments to elucidate the truth of this 

 matter, has shown that under the influence of this convergence the behaviour 

 of the chlorophyll grains is very marked. In a species of Mesembryanthemum 

 there are special lens-cells which are equally well developed on both upper 

 and lower surfaces. In Garrya elliptica, too, there are special lens-shaped 

 thickenings of the cuticle on both surfaces. It is worthy of note, as 

 supporting the above hypothesis, that, so far as observed at present, 

 epidermal cells of long focus are associated with long palisade cells, and the 



[E. Step. 



FIG. 118. LECANORA PARELLA. 



A Lichen growing on a rooflng tile. In such a situation it is evident 



that the whole of its nourishment must be obtained from the 



atmosphere. Natural size. 



