FEBRUARY 28, 1907. 



The Weekly Rorists' Review* 



1083 



the immeaiate advantage is not obvious, 

 especiaJlj since most of its ways and 

 method ar6 beyond the comprehension 

 of the onlookers. Yet pure science work 

 is a necessary precursor of applied 

 science. Again, in the course of purely 

 scientific investigations, we often meet 

 with facts, which when properly analyzed 

 throw considerable light on questions of 

 practical importance. Or at least give 

 us suggestions and new ideas for further 

 experimentation. 



The Plant's Relation to Light. 



A question that has received much 

 commentation in recent years is that of 

 the advisability of shading plants, like 

 roses and carnations, in greenhouses in 

 summer. Some hold that all shade is 

 not only superfluous, but even injurious, 

 while others maintain that shade of some 

 kind is necessary at least during the 

 bright months. To arrive at a clear 

 understanding of the problem let us con- 

 sider the plant in its relation to light. 



About fifty per cent of the dry weight 

 of plants is carbon. All of the carbon 

 is obtained from the atmosphere, which 

 contains about 2.94 parts per 10,000 by 

 volume; i. e., in 10,000 cubic leet of air 

 there are about three cubic feet of car- 

 bon dioxid. The carbon dioxid is taken 

 into the green parts of plants, mostly 

 the leaves, and is there worked over into 

 organic matter. This process, however, 

 takes place only in the light. Tlie energy 

 for the chemical transformations of the 

 carbon dioxid into starch and sugar in 

 the leaves under natural conditions is 

 furnished entirely by the sunlight. "Very 

 feeble light is sufficient to cause the 

 process of carbon assimilation to begin. 

 The rate of assimilation increases with 

 intensity of illumination until a certain 

 maximum of illumination is reached, 

 after which no further increase of as- 

 similation is observed. Keeping this in 

 mind, that the amount oft carbon dioxid 

 in the atmosphere is exceedingly small, 

 and that sunlight is the chief factor con- 

 cerned in its assimilation, I will now give 

 the results of some recent work of two 

 English investigators, Brown and Es- 

 combe, bearing on the question of the 

 influence of intensity of illumination on 

 the assimilation of plants. 



Some Interesting Experiments* 

 It is not necessary to go into the de- 

 tails of the manipulations except to say 

 that the leaves while still attached to the 

 ])lants were enclosed in air-tight glass 

 cases through which a stream of air was 

 draAvn by means of tubes. By determin- 

 ing the amount of carbon dioxid in the 

 entering and also in the outgoing ftir, 

 it is an easy matter by subtraction to de- 

 termine the amount assimilated by the 

 leaf. 



In the first place, it was found that 

 when leaves were shaded by a thin cloth 

 screen the assimilatory process always 

 responded to an artificial increase in the 

 amount of carbon dioxid in the air en- 

 tering the case. In other words, even 

 when the illumination was reduced the 

 plant was still able to assimilate more 

 carbon dioxid than is normally present 

 in the air. That is to say that even with 

 moderate illumination the amount of 

 light which a plant receives is still in 

 excess of the amount which the leaf re- 

 quires with the normal carbon dioxid 

 content of the air. 



In the second place, another series of 

 experiments was carriea out by Brown 

 and Escombe in which they determined 

 directly the amount of carbon dioxid as- 



Albert T. Hey. 



(President State Floiiats' Association of Illlonols.) 



similated by shaded leaves. In this case 

 the leaves of the ordinary nasturtium, 

 Tropa-olum majus, a sun-loving plant, 

 were used. The leaves to be experi- 

 mented upon, while still attacherl to the 

 plant, were enclosed in a square case 

 with glass sides as before. These leaves 

 were exposed under a screen which cut out 

 about two-thirds of the solar radiation. 

 By a very ingenious method a known por- 

 tion of the remainder of the light was 

 cut out. This was effected by means of 

 revolving discs from which radial sectors 

 had been cut. If a disc from which, for 

 instance, one-half of the area had been 

 cut away was revolved over the leaf it 

 is evident that only one-half of the light 

 can reach the leaf. The following table 

 gives the amounts of carbon dioxid as- 

 similated per hour per unit surface of 

 leaf under different strengths of illum- 

 ination. It must be kept in mind that 

 all the leaves were under a canvas screen 

 which cut out about two-thirds of the 

 light which was then further reduced by 

 the revolving sectors. 



two-thirds of the illumination was cut 

 off by a screen and one-half of the re- 

 mainder was cut off by the revolving 

 sectors the amount of carbon dioxid as- 

 similation was not decreased but was 

 even slightly increased. Not until three- 

 fourths of the light coming through the 

 screen was cut off was there any reduc- 

 tion in assimilation. This means that 

 one-sixth of the light of a clear summer 

 sky is still sufficient for the needs of our 

 ordinary plants and not until the light 

 is reduced to about one-twelfth do thej 

 begin to suffer. 



Now I wish to correlate with these 

 experiments another set of observations 

 and experiments from an entirely differ- 

 ent source, but having a bearing on this 

 sul)jcct. 



Schander's Experiments. 



The use of Bordeaux mixture as a 

 fungicide is well known. The spraying 

 of plants such as apple trees, potatoes, 

 grapes and others is a matter of com- 

 mon practice. Soon after the use of 

 Bordeaux mixture became general it was 

 observed that the mixture had secondary 

 effects on plants not due to its fungicidal 

 action. The plants thus spray^ re- 

 mained green and retained their leaves 

 longer than unsprayed plants. The leaves 

 were of a deeper color and thicker than 

 those of unsprayed plants. This was a 

 matter of common observation both in 

 this country and in Europe. Much work 

 was done to determine the cause of this 

 effect, and it was generally attributed 

 to the stimulating action of the mixture. 

 Kecently the whole subject has been in- 

 vestigated by Schander, a German in- 

 vestigator, whoso results I will give 

 briefly. 



After eliminating bv careful experi- 

 mentation the possible stimulating effects 



