IRRITABILITY. 475 



upon which the seed has germinated, and he recalled and 

 confirmed Spallanzani's observation that the sporangiferous 

 hyphae of Moulds grow perpendicularly outwards from their 

 substratum. He proved, moreover, that the influence of the 

 substratum on the growing organ is not due to the mere 

 physical attraction between them, that is the result of a 

 stimulating effect on what he terms the "nervimotility" of the 

 organ. 



Dutrochet's observations on Moulds have been comparatively 

 recently confirmed by Sachs, who has found that when Mucor- 

 spores are sown on a suspended cube of bread, in darkness, 

 the sporangiferous hyphae developed on the upper surface of 

 the cube grow vertically upwards, whereas those developed on 

 the lateral and inferior surfaces grow for a time perpendicularly 

 outwards, but subsequently curve upwards in consequence of 

 negative geotropism. The perpendicular direction of growth 

 of the hyphae growing on the lateral and inferior surfaces of 

 the cube is maintained, in opposition to geotropism for a time, 

 until they attain a certain length, and then geotropic curvature 

 takes place. The directive influence of the substratum thus 

 appears to be limited to a certain distance from its surface. 

 Mycelial hyphae were only developed on the lateral and 

 inferior surfaces of the cube of bread, in no case on the upper 

 surface ; those that projected from the lateral surfaces behaved 

 just like the sporangiferous hyphae, only they subsequently 

 curved downwards ; those which projected from the inferior 

 surface grew straight downwards. Similar results were obtained 

 by Sachs with Pilobolus and Coprinus. 



From these observations it is clear that the substratum 

 exerts, for a time at least, a directive influence on growing 

 organs, which is spoken of as Somatotropism ; and growing 

 organs may be classified according to their response to this 

 influence, into two classes, the positively and the negatively 

 sornatotropic ; the former are represented in the above obser- 

 vations by the radicle of the Mistletoe, and the mycelial hyphae 

 of the Fungi, which tend to grow perpendicularly inwards into 

 the substratum ; the latter, by the primary stem of the Mistletoe, 

 and by the sporiferous hyphae of the Fungi. We may go so 



