HELIO TR OPISM. 675 



case a modified curvature. This is tlie one way wliich. a plant lias 

 of reacting to the external world. Motile organisms, as, for in- 

 stance, the microscopic swarm-spores of Hmmatococcus or Botry- 

 dium — a couple of fresh-water algse — may seem to stretch the 

 strict interpretation ; but their movements may be considered un- 

 der the law if one remembers that they are free, solitary cells, and 

 must act accordingly. 



Twining plants are, perhaps, the most interesting examples of 

 an-heliotropic irritability, for their habit of growth — by no means 

 leaving the instinctive circumnutation of the tip out of account — 

 is a manifestation of insensibility to light. The morning-glory is 

 a perfect example. Regardless of the sun, it twines regularly 

 along its support, never for a moment being deflected or turned 

 aside through conditions of unequal illumination. Of the same 

 thing the wistaria is an equally instructive illustration. It may 

 be safely presumed that, originally, twining plants were not twin- 

 ing plants at all, but were creeping in their habits ; and from this 

 it seems probable that heliotropism was once present, but is now 

 lost. In its first appearance the habit of twining must have been 

 accidental, and just how heliotropic tendencies were overcome by 

 the newly developed trait is difficult to explain. It is interesting to 

 notice, however, that the shoot of the morning-glory, when it first 

 peeps from the ground, is distinctly heliotropic, and this must be 

 considered an embryonic feature significant precisely as the bran- 

 chial development of the foetal mammal is significant — that is, 

 there is indicated by it a line of descent. 



A distinction must first be made between the periodic move- 

 ments of leaves and stems and the true heliotropic movements. 

 As pointed out by Dr. Julius Sachs, the first — such as the well- 

 known phenomenon of sleep — are dependent upon the intensity of 

 illumination, while the second are almost entirely due to the direc- 

 tion from which the rays chance to be falling upon the plant. It 

 will be indispensable to a clear comprehension of what true helio- 

 tropism is to speak somewhat generally of light-action in vege- 

 table physiology. 



There is certainly no more important agent in the whole system 

 of Nature than light. It is only in light that green plants can 

 form their chlorophyl, and, since this chlorophyl is absolutely 

 essential in the assimilating processes, its importance can be con- 

 ceived. Even chlorophylless plants — the fungi and slime-molds 

 — and all animals are indirectly dependent, as is well known, upon 

 the possibility of chlorophyl-formation. The whole organized 

 world, then, depends upon light as one of its essentials, for undu- 

 lations of the ether are necessary to the well-being of all proto- 

 plasm. But not only does protoplasm react to these undulations, 

 in very many instances, by the elaboration from itself of the 



