iQ2o] . CURRENT LITERATURE 187 



the observer is easily able to estimate 10 different degrees of covering. From a 

 record of the numbers representing these degrees of covering the areal per- 

 centages of the different species are readily established. 



A summary of the methods employed, and a classification of vegetation 

 upon the basis of life-forms and leaf -sizes, completes an article rich in sugges- 

 tions to the ecologist seeking more accurate methods. — Geo. D. Fuller. 



Susceptibility gradients. — Following his demonstration of axial metabolic 

 gradients in animals and their relation to the course of development and 

 individuation, Child entered upon a study of axiate plants, particularly the 

 algae. His first paper 14 on axial gradients in algae appeared several years ago. 

 His interesting and valuable observations 15 have been extended to include 

 a considerable number of new forms, and the results are sufficiently uniform to 

 warrant the general conclusion that plants and animals are essentially similar 

 in respect to these axial susceptibility gradients. 



Twenty-five species have been studied, 14 of which were considered in the 

 earlier paper, and all of them show r an axial gradient in susceptibility to injury 

 and death from such agents as KCN, alcohol, ether, HC1, HgCl 2 , CuS0 4 , 

 neutral red, temperature, etc. When killing concentrations are used, death 

 occurs first in the apical region and proceeds basipetally in each axis. The 

 susceptibility gradient is a general indicator of metabolic rates, death occurring 

 soonest in the most active protoplasm. The susceptibility gradient is rather 

 easily altered or reversed by external conditions, by advancing age, physio- 

 logical isolation of cells and branches, and other factors. The ease with which 

 such reversals occur indicates in some degree the sensitiveness of species. 



He finds 16 that the unicellular and multicellular hairs, either branched or 

 unbranched, which occur on some algae, possess the same kind of axial gradients 

 as the main axis. In such forms as Fuctis and Castagnea, in which the hairs 

 have basal growth, the gradient is acropetal; but whenever the hairs grow 

 apically the normal gradient is basipetal. Reversals may be induced in these 

 hairs, also, especially with low concentrations of the susceptibility reagents. 

 In some cases the agent may reverse the susceptibility to itself, or one agent 

 may reverse the susceptibility to another agent. These results indicate clearly 

 that hairs represent physiological axes, and the gradient of susceptibility 

 appears to be one of the aspects of physiological polarity in all axes. When 

 the axial gradients are reversed, these hairs often separate into their component 

 cells, or the hairs drop from the main axes. Loss of hairs in laboratory material 



14 Child, C. M., Axial susceptibility gradients in algae. Bot. Gaz. 62:89-114. 

 1916. 



15— ^ Further observations on axial susceptibility gradients in algae. Biol. 



Bull, 31:419-440. 1916. 



16 , Susceptibility gradients in the hairs of certain marine algae. Biol. 



Bull. 32:75-92. 1917. 



