64 BIOLOGY: GENERAL AND MEDICAL 



Among the lowest animals and plants, especially 

 those that are free and motile, geotropic reactions are 

 either undetermined or vague. 



Among the fungi, however, with increasing complexity 

 of structure, there is an increasing disposition toward a 

 definite adjustment of the organism with reference to the 

 earth's surface. Thus among the moulds, Aspergillus 

 and Penicillium most commonly direct their sporangia 

 upward, and among hyphomycetes in general the aerial 

 hyphae grow perpendicularly to the plane of the earth's 

 surface. 



Among the Basidiomycetes, the Polyphoreae and 

 Agaricineae, which include the mushroom and toad- 

 stool-like organisms, tend toward a perpendicular posi- 

 tion, the pileus spreading in a plane corresponding to 

 that of the earth's surface. 



The general tendency of the higher cryptogams is to 

 maintain a line of growth perpendicular to the plane 

 of the earth's surface. 



The same general tendency pervades pretty much 

 the whole group of phanerogams. 



In considering the geotropic reactions of plants, it 

 becomes necessary to speak of positive, negative, and 

 lateral geotropism and to make brief mention of diageo- 

 tropism. 



In such plants as show typical geotropic reactions, 

 the stem which rises vertically, that is, perpendicularly 

 to the plane of the earth's surface, is negatively geotropic; 

 the branches that extend from it in a plane more or less 

 parallel with the earth's surface, diageotropic, and the 

 tap-root, that descends perpendicularly to the earth's 

 surface, positively geotropic. 



This behavior takes place regardless of the sources 

 of light and heat and in obedience to the force of gravity 

 alone. 



Knight found that if germinating seeds were fastened 

 to a rapidly revolving wheel moving in a vertical plane, 

 by which the force of gravity was set aside, the direction 



