202 



The Plant World. 



initiates a decided advance over the plan of describing the habi- 

 tat of a species as sunny, partially sunny, shaded, or deeply 

 shaded. Every such extension of our knowledge of plants makes 

 one long for an ecological Bora of his own locality. Impracticable 

 for extensive regions, such a flora could be worked out for a limit- 

 ed area in such a way as to include most of the things best worth 

 knowing about the commoner seed plants treated. It might 

 cover their light, heat, and moisture requirements, their relation 

 to soil composition, to animal and plant parasites, and to pollin- 

 ating insects. Much attention should be paid to the influence 

 of these factors of the environment in causing variations. 

 Who now knows all of these details that might be learned about 

 any single species? 



One of the chief difficulties in the way of getting an accurate 

 knowledge of the light-relations of plants is that it is very diffi- 

 cult to determine with reasonable accuracy the amount and kind 

 of light which they daily and yearly receive. Notwithstanding 

 the existence of good photometers for the physicist and so many 

 kinds of sensitive paper which will give some kind of measure- 

 ment of relative light intensity, there is no instrument which 

 measures illuminations as promptly and accurately as the thermo- 

 meter measures relative temperatures. As is well known the 

 light from different portions of the spectrum affects the plant 

 very differently. Apparently, transpiration is most favored 

 bv blue light, heliotropic movement by light from the portion of 

 the spectrum between violet and ultra violet, photosynthesis 

 bv light from the red to the yellow portion. Blue and ultra 

 violet light are necessary to the healthy growth of green plants, 

 serving to prevent excessive growth and etiolation. It is evi- 

 dent then, that sensitized paper which is acted on by only a few 

 kinds of rays is not a satisfactory measure of the value of any 

 given illumination for plants. If, however, we know that the 

 light which they receive in any particular case is merely weak- 

 ened sunlight, with the several regions of the spectrum all pro- 

 portionally represented, we may fairly make use of ordinary 

 photographic paper for our measurements. It will then be 

 necessary to standardize the paper in some way in order to know 

 what fractional part of the full sunlight the area, plant, or part 

 of a plant under consideration is at the time of observation re- 



