SUSCEPTIBILITY GRADIENTS IN THE HAIRS OF 

 CERTAIN MARINE ALG/E. 



C. M. CHILD, 

 WITH FIVE FIGURES. 



The colorless hairs or capillary branches so common on the 

 red and the brown algae are in many cases beautiful objects for 

 the study of the axial susceptibility gradients. A few data 

 obtained during the summer of 1915 at Woods Hole have already 

 been published (Child, 'i6a). The present paper records the 

 result of further investigation during the summer of 1916 on the 

 physiological polarity of these hairs and includes data on addi- 

 tional species, experimental results on modification of the gra- 

 dients which answer certain questions raised by the earlier work, 

 and also a correction of the earlier observations concerning one 

 species. 



Thus far hairs of the following three types have been examined : 



Unbranched unicellular hairs, Ceramium, Chondrus, Agard- 

 hiella. The hair is a very slender and delicate outgrowth, 

 sometimes reaching a length of several millimeters, but uni- 

 cellular. 



Unbranched multicellular hairs with basal vegetative tip, 

 Fucus, Castagnea. 



Branched, multicellular hairs with apical vegetative tip, 

 Chondria, Polysiphonia, Griffithsia. 



Although this grouping of species according to the form of the 

 hairs does not represent the taxonomic order, it seems the most 

 satisfactory for present purposes since only the hairs are to be 

 considered. In most species examined the hairs are so extremely 

 sensitive that conclusions concerning the existence of a gradient 

 and its direction can be safely drawn only from plants in the 

 best possible physiological condition, collected with the minimum 

 of handling and examined at most within a few hours after col- 

 lection. Within the first few days in the laboratory, even in 

 running water, the hairs usually die and often drop off, although 



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