BELIO TR OPISM. 68 1 



this one end. Leaf -formation will be scanty ; assimilation will be 

 suspended ; and the whole organism will reach out for the sunlight, 

 as thousands of generations had done before its own life. That 

 fungi of the mushroom type — needing no light, for they make no 

 chlorophyl — reach upward too — and it is undeniably true that 

 they elongate more rapidly in darkness — is to be considered as 

 evidence of descent from an alga stock, and this is rendered prob- 

 able by morphological as well as by this interesting physiological 

 consideration. Heredity may come into play here as well as it 

 does in the case of the moss antherozoids, which are attracted by 

 the archegonium, or in the case of the fish-mold zoospores, which 

 swim toward decaying fish or putrid extracts of meat. 



From all this, the meaning of heliotropism in the natural order 

 of things becomes apparent. The phenomena which have been 

 studied fit into the evolution theory as if made for the theory and 

 not the theory for them. Plants which must have light to live 

 are impelled toward this light by their own conditions of struct- 

 ure. The reaching upward is sometimes almost instinctive — 

 almost conscious, one might fancy. Knight observed a vine-leaf 

 try first one way and then another to reach the position of best 

 illumination — a transverse one, which is now considered to be a 

 result of the palisade structure, and not of a peculiar kind of 

 irritability. Dutrochet noticed the tendril of a pea trying to avoid 

 the light, and it finally seemed to send an impulse down to the 

 petiole, and this bent backward. The question of resistance is 

 probably, however, the only one which needs to be considered as 

 modifying plant-action in such instances. 



Climbing and twining plants, as Darwin observed, have lost 

 their heliotropism because they would be pulled away from their 

 supports if they always followed the sun. For the same reason 

 tendrils, aerial roots, the suckers of Parthenocissus quinquefolia — 

 the Virginia creeper — are, considered as separate organs, aphelio- 

 tropic rather than the reverse. Carnivorous plants, which at least 

 partially depend for sustenance upon a peculiar position of leaves 

 and stem, and which have less need of light for assimilation, have 

 also lost their powers of* response to the heliotropic stimulus. It 

 must not be supposed, however, for a moment, that heliotropic 

 irritability is not present. It may be there, and well developed 

 too ; but inhibited by heredity, by growth, by environment. Just 

 as the compass-plant when grown in darkness allows its leaves to 

 adopt the horizontal position, so does the Venus's-flytrap when 

 growing normally, and it is probable that, if generation after gen- 

 eration of compass-plants could be grown in the darkness, the erect 

 position of the leaves would permanently disappear. Just so the 

 Venus's-flytrap has lost the power of responding to its heliotropic 

 tendency, through its carnivorous habits. Vines, indeed, thinks 



