BOOKS AND CURRENT LITERATURE 195 



6. The formation of oxalic acid from acetaldehyde, as shown in the 

 above diagram, is a possible explanation of the occurrence of large 

 amounts of this acid in succulents, for although oxalic acid is easily 

 decomposed in sunlight, it is, nevertheless, accumulated in the plant 

 cells by the daily formation of stable compounds with calcium. 



7. Malic, glycollic, and acetic acids all produce formaldehyde under 

 the influence of light and hence the occurrence of formaldehyde in plant 

 tissues gives no evidence either for or against the Baeyer assimilation 

 hypothesis. 



The paper closes with an interesting comparison of the results of this 

 work with those obtained by Borowikow. The latter found that growth 

 rate in plants is influenced in the same manner by the presence of acids, 

 bases and salts as is the rate of swelling of colloids and deduced the 

 generalization that growth is caused by the swelling of the colloids 

 within the cell. Now, since the acidity of plants decreases in light and 

 since a decrease of acidity would decrease the rate of colloidal swell- 

 ing, a cause for the retardation effect of light on growth is found. — 

 Edith B. Shreve. 



Mycorhizas.— This study^ is a distinct contribution to our knowl- 

 edge concerning mycorhizas that has been gained from the sixty or 

 more papers that have appeared on some phase of the subject. In 

 addition to the ectotrophic and endotrophic mycorhizas the author 

 reports a condition not previously recorded in which both forms appear 

 on the same rootlet and are caused by the same fungus. For this con- 

 dition he uses the term heterotrophic. Several different species of 

 mushrooms may form mycorhizas on the same trees, the converse of 

 which had previously been known. However, not all mycorhizal fungi 

 are capable to relating themselves to all species of mycorhizal trees. 



The fact that infection takes place through the root hairs in the 

 formation of endotrophic mycorhizas is confirmed. In the case of 

 infection in the formation of ectotrophic mycorhizas the fungous fila- 

 ment first penetrates the outer portion of the epidermal wall of the roots, 

 dissolves the middle lamellae, and by continued growth a complete 

 mantle is formed over the rootlet. 



The hypothesis that ectotrophic mycorhizas represent symbiotic 

 associations in which the fungus serves as the conveyor of salts from 

 the soil to the root has been quite generally accepted. McDougall 



iMcDougall, W. B., On the Mycorhizas of Forest Trees. Amer. Jour. Bot. 

 1: 51-74, pis. 4-7, fig. I, 1914. 



