342 BOTANICAL GAZETTE [may 



form 



While the book is a good one, we doubt whether, considering its handicap, 

 there is room for it in the German market in competition with the other excellent 



texts which are there available. — C. R. B. and C. J. C. 



The lighting of plants 



For almost the entire period of his half-century of fruitful activity as a physiolo- 

 gist, WiESNER has been occupied with the relations of light to the functions of 

 plants, and for the last fifteen years he has devoted his attention particularly to the 

 relation of light to the plant as a whole. This relation is expressed briefly in the 

 word LichtgentisSy a word impossible of translation. On that account he has 

 suggested the less happy photolepsy (in English "light-catching"); for ''lepsy" 

 lacks the idea of limits set by satisfactoriness that is involved in Genuss, Wiesner 



an easily comprehensible synopsis of the 

 results of his long study, carried on in many and diverse regions.^ It would be a 

 mistake, however, to suppose that the book consists of extracts from earlier works, 

 on which confessedly it is based. In it a considerable number of researches are 

 now first pubUshed, especially those on the specific green of the leaves of woody 

 plants as related to their illumination. Also new is the attempt to present a 

 physiological analysis of Lichtgenuss. 



The Lichtgenuss (L) of a plant may be expressed either relatively or abso- 

 lutely. Relatively it is that part of the total daylight which reaches it. Thus if i 

 be the intensity of light reaching the plant taken as unity, and / the intensity of 

 the total daylight expressed in that unit, then L-=i/I. Absolutely L may be 

 expressed m any convenient photometric units, and to photometric methods the 

 author devotes the first chapter of the book. These, convenient and adaptable as 

 they are, leave still to be desired a method which will reveal more accurately the 

 photosynthetic value of the light which falls on a plant. 



The second chapter analyzes the daylight, consisting as it does of direct and 

 reflected sunlight in varying proportions, directions, and intensities, and shows 

 how total light may be reckoned and graphically represented through the course 

 of a day or a longer period. In the third chapter the illumination of plants is dis- 

 cussed, showing how it is affected by all sorts of conditions, and how plant form 

 is influenced by the lighting. The fourth chapter is devoted to specific observa- 

 tions upon the photolepsy of various types of plants in their habitats; the fifth 

 treats of the constancy or variability of photolepsy in different stages of develop- 

 ment, and the optima thereof; the sixth discusses the dependence of photolepsy 

 upon latitude and altitude, and includes particularly the author's observations in 

 this country in 1904; the seventh deals with various sorts of leaf-fall; and the 



pomts 



mit 



)er Lichtgenuss der Pflanzen. Photometrische und physiologische 

 besonderer Rucksichtnahme auf Lebensweise, geographische Ver- 



breitung und Kultur der Pflanzen. 8vo. pp. viii-^322. figs, 25. Leipzig: Wilhelm 

 Engelmann. 1907. Mg, 



