The Light-Requirement of Plants. 



203 



ceiving. These questions of photometric methods and of the 

 nature and intensity of daylight are fully discussed in the first 

 two sections of Wiesner's work. * 



One of our author's important contributions to the general 

 subject of light-relations of plants, discussed in detail in earlier 

 works and briefly outlined in his Lichtgenuss, is the classification 

 of leaves with reference to their utilization of light. Those 

 which show no fixed relation to the incidence of light, such as 

 pine needles, are classed as aphotometric. Those which show 

 some relation, either fixed or adjustable, are photometric. Of 

 the latter class there are two divisions, eu photometric leaves, and 

 pan photo metric leaves. Good examples of the euphotometric 

 class are those in the interior of the crown of most deciduous 

 trees. These exercise the greatest economy of light and are 

 so placed as to receive as much diffused light as possible. If 

 thev are borne by plants which grow in the shade of forest trees 

 (not on the margins of woods) they extend almost horizontally 

 to get the maximum amount of the light which filters through 

 the leaves above them. Panphotometric leaves show the most 

 complex adaptations of any, since they can ward off some direct 

 sunlight by exposing their edges to it, but offer a much greater 

 surface for the absorption of diffused light. Such leaves as those 

 of Robinia Pseudo-Acacia and Phciscolus , which meet the vary- 

 ing illumination of different periods of the day with rather 

 prompt changes of position are in the highest degree panphoto- 

 metric. 



For botanists in general the most suggestive part of Wies- 

 ner's book and the one to which reference will oftenest be made, 

 is the fourth section, which deals with the light-requirement of 

 the plants of some of the principal ph\ siographical regions, as 

 steppes, deserts, tundra and tropical forests. It also takes up 

 the light-relations of a few of the great groups of plants and 

 gives in a good deal of detail the author's values for the light- 

 requirement of many species, mostly of seed plants. 



The lowest light-requirement of any kind of plant reported 

 on (which needs light at all) is that of some lichens. These 

 grow, but do not fruit, in lower Austria, with a light-require 



♦The reader who is desirous of Icoking up lie lilcialvxe <f tbif mtljecl will fir.d 

 sufficient references for the purpose in Meffer's I hysiolcgy of Plants, Clarendon Press, 

 oxford, 1900-1906, and in Burgtr^ttin's Die Transpiration der Pflanzen, Jena, 1904. 



