164 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



flowered; bracts, green ovate; sepals, yellowish-white; petals, bright 

 orange. — G. H. 



Plant-associations, Cycles of. By Professor H. 0. Cowles 

 (Bot. Gaz. pp. 161-83; March 1911).— This article, ''The Causes of 

 Vegetable Cycles," is the address delivered as retiring President to the 

 Association of American Geographers in December 1910. 



The author gives a short historical resume, including some remarks 

 on the changes oi flora in Carboniferous, Permian, Jurassic and Cre- 

 taceous times, which he is inclined to attribute to changes in humidity 

 as well as alterations in the temperature. Most of the article deals 

 with changes brought about by plant and animal agencies. Of these 

 the " humus complex " is fully described. The most important effect 

 of humus is, in his view, an increase in soil moisture on uplands, and 

 a decrease in depressions. The increase in soil organisms (such, e.g., 

 as the saprophytic fungi, which makes the growth of beech forest 

 possible) is also an important effect of humus accumulation. Another 

 factor is soil toxicity, such as is shown by the detrimental substances 

 in bogs. Other effects of humus are the increased amount of available 

 plant food and differences in the temperature and aeration of the soil. 

 The effect of Shade, of Plant Invasions and of Man are shortly de- 

 scribed. The author points out that within one great climatic cycle 

 there may be many cycles of " erosion," each with its own vegetative 

 cycle. There is also a short bibliography giving a few recent American 

 papers on the subject. — G. F. S. E. 



Plant Production, School Exercises in. D. J. Crosby {U.S.A. 

 Dep. Agr., Farmers' Bull. 408, p. 48; Sept. 1910; 39 figs.).— This 

 bulletin is a reproduction of one written for elementary school teachers. 

 The exercises are suitable for the upper standards of an English school, 

 and provide excellent material for a lesson course to supplement 

 school-gardening. 



The exercises are grouped into two divisions. The first group deals 

 with the plant itself, and the second part deals with the influence of 

 the environment. In all there are forty-six exercises. Some of them 

 can be met with in various English text-books on Nature study, but 

 I have never yet seen a book where most of these exercises are given. 



The author makes no claim to originality in these exercises, though 

 some of them are rarely met with in text-books written for teachers in 

 our schools. 



Much that is valuable from a teacher's point of view can be obtained 

 from the bulletin.— TF. W. 



Platycerium, Grape Sugrar as an Excretion in. By E. 



Dummer {Ann. Bot. xxv. pp. 1204-5; Oct. 1911). — Platycerium grande 

 is shown to possess nectar-excreting tissues. In the early hours of 

 the morning (6 a.m. to 9 a.m.), in the tropical ferneries at Kew, the 

 lower surfaces oC tlie young barren fi-oiuls, or "mantle leaves," are 

 copiously .studded towards tlieir (ips with drops of liquid, which 



