1914] CURRENT LITERATURE 451 



progressively hairy from the base to the top. On non-flowering shoots, how- 

 ever, the later leaves, like the earlier, are glabrous. On partially hairy leaves 

 the terminal leaflet is invariably the most hairy, and there is a regular decrease 

 in hairiness downward. Although it is more conspicuous in Spiraea Ultnaria, 

 this seasonal periodicity in hair formation was found in many other plants. 

 The lower glabrous leaves have the structure of shade leaves, whereas the 

 upper leaves may be called sun leaves. While the xeromorphic upper leaves 

 have more stomata per unit area, the size of the aperture is less. Correspond- 

 ence is found between hair production and evaporation curves, both seasonally 

 and in height above the soil. Spiraea is found to be less plastic, however, in 

 respect to hairiness than are various plants previously reported (e.g. Mentha 

 aquatica and Polygonum amphibium) . It was found impossible to make the 

 early leaves hairy or the later leaves smooth. The first parts to wither are 

 those that are hairy. As to cause, hair production may be related to heredity, 

 individual variation, or external factors. Of the latter it is believed that 

 a diminished water supply is the most important. Yapp believes that Spiraea 

 Ulmaria is becoming fixed as to the presence or absence of hairs; already the 

 glabrous condition of the early leaves is a fixed character, and the hairiness 

 of the later leaves is tending to become fixed. He concludes that xeromorphy 

 is of physiological utility to those bog and marsh plants which exhibit it, but 

 that no one factor can be singled out as of sole importance in determining 

 xeromorphy. The physiological dryness of the soil is important, but it does not 

 explain everything. For example, the presence of hygrophytic and xerophytic 

 species side by side is likely to be explained by differences in transpiration, the 

 xerophytic species generally being more exposed to severe transpiration. 

 H. C. Cowles. 



Low temperature and plant tissues. — Chandler* has treated in consider- 

 able detail the killing of plant tissues by low temperature. He discusses the 

 nature of injury by low temperature, giving a good summary of the literature 

 relating to this subject. In his experiments he has shown that the killing 

 temperature of many plant tissues may be reduced by means which increase 

 the sap density; for example, intake of potassium chloride, glycerine, etc., or 

 partial withholding of water. Rapidity of thawing had little effect on the 

 amount of killing at a given temperature in case of all tissues experimented 

 upon except that of ripe fruit of apple and pear. Previous exposures to low 

 temperatures lowered the death point. In case of hardy winter buds and wood, 

 a rapid fall of temperature increased the severity of injury. In fruit trees 

 during early winter the wood at the base of the trunk and at the crotches of 

 rapidly growing branches was found most susceptible to injury. At the stage 

 of most complete maturity, the pith cells and fruit buds were most tender of 

 all the above ground tissues. The hardiness of root tissue was found in general 



5 Chandler, W. H., The killing of plant tissues by low temperature. Research 

 Bull. Agric. Exper. Sta. Univ. Mo. no. 8. pp. 143-309. Jigs. 3. 1913. 



