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LECTURES TO SCIENCE TEACHERS. 



geotropic. Geotropisin is by no means confined to roots 

 and stems, nor is its negative or positive character in- 

 separably connected with the morphological nature of the 

 organs of plants. It is true that all primary roots are 

 positively geotropic, as well as most secondary roots, but 

 so also are many leaf -bearing axes, and even leaves, as in 

 the case of the cotyledonary sheaths of Allium and other 

 Monocotyledons ; and to these we may add the lamellae 

 forming the hymenium of mushrooms so that we have 

 an assemblage of structures of the most varied morpho- 



FJG. 2. 



logical value, all of which exhibit positive geotropism. On 

 the other hand, all erect leafy axes, petioles, stipes of mush- 

 rooms, conidiophores of moulds, are negatively geotropic. 



We now come to an apparatus (No. 3938) which is very 

 much more complicated than any of those with which we 

 have already become acquainted. It is an apparatus 

 exhibited and constructed by Herr E. Stohrer, of Leipsic, 

 for registering the growth of plants (Fig. 2). A fine 

 thread is attached by one end to the growing internode of 



