SUSCEPTIBILITY GRADIENTS. QI 



of the fronds bearing hairs show a lower susceptibility than other 

 parts (Child, 'i6b). The hair is an axis arising from a cell of a 

 previously existing axis. Like any other physiological axis it 

 represents a metabolic gradient and originates as such a gradient 

 in a cell which, though apparently specialized in certain respects 

 is still capable of undergoing growth and division. In species 

 where the hairs are not extremely sensitive to laboratory con- 

 ditions it will probably be possible to control their occurrence 

 and development by controlling the metabolic condition of the 

 plant and in this way to learn something of their physiological 

 relation as axes to the other axes of the plant. 



SUMMARY. 



1. The unbranched unicellular hairs of Cemmium, Chondrus 

 and Agardhiella, the unbranched multicellular hairs of Fucus and 

 Castagnea and the branched multicellular hairs of Chondria, Poly- 

 siphonia and Griffithsia all show an axial gradient in suscepti- 

 bility to various agents. 



2. In the hairs of Fucus and Castagnea in which the vegetative 

 point is basal the normal hair gradient is acropetal while in the 

 other species where growth is primarily apical the gradient is 

 basipetal, i. e., in all cases the region of highest susceptibility is 

 the region of the "growing tip." 



3. Obliteration or the reversal of the susceptibility gradients 

 has been observed or induced in the hairs of Ceramium, Fucus, 

 Castagnea and Chondria and to some extent in those of Grif- 

 fitlisia. These changes in the gradient are dependent on the 

 differential susceptibility to various depressing agents and 

 conditions, which is a feature of the normal gradient. In general 

 a high concentration or intensity kills so rapidly that reversal of 

 the gradient cannot be demonstrated, but with lower concentra- 

 tions or intensities the original differences in susceptibility may 

 be obliterated or reversed before death occurs. In some cases a 

 particular agent may reverse the susceptibility gradient to itself, 

 and in others a reversal of the gradient produced by one agent 

 may be demonstrated by the differential susceptibility to another. 



4. The hair, like the vegetative portions of the plants, repre- 

 sents a physiological axis and the susceptibility gradient is one 



