i9o8] CLAPP— STUDY OF TRANSPIRATION 267 



to objection in that it is not kept in greenhouses and must be grown 

 expressly several weeks before needed for use in the classroom' 

 Vicia Faba, Lupinus albus, Lycopersicum esculent iimj Ipomoea pur- 

 purea^ and Cucurbita Pepo, although they show a high rate of transpira- 

 tion, are open to the same objection. The first choice falls, then, in 

 order of respective excellence to Chrysanthemum frutescenSy Tropaco- 

 lum majus, Pelargonium domesticum, Fuchsia speciosa, Senecio 

 PelasUiSy Senecio mikanioides, Pelargonium zonale, Hcliotropiu m 

 peruvianum, and Pelargonium peltatunu They are best for two 

 reasons: their first value lies in the fact that they lose large amounts 

 of water; the second, and no less important, is that in addition to 

 their being easily obtained at any time of year from the greenhouse, 

 they are grown in many homes. An objection to Euphorbia pukJier- 

 rima and Impatiens Holstii^ moderate transpirers, is that they lose 

 many leaves just at maturity; this is true also of the heliotrope. Of 

 least value are Primula sinensis, Coleus Blumei, Hedera Helix, Ces- 

 Irum elegans, Begonia argentea, and Tradescantia zebrhia, which, 

 though common, have a very low rate of transpiration. 



In addition to the value given by the tables for the different kinds 

 of plants, it will be of some interest to note the average transpiration 

 for these plants taken collectively. The result may be considered a 

 general expression of the amount of transpiration of ordinary plants 

 growing in any greenhouse. This quantity is 48.732 or in round 

 numbers about 50 grams per hour per square meter of surface for the 

 day time, and 8.898, or in round numbers 10 grams for the night 

 about five times as much on the average per hour per square meter 

 by day as by night. 



NoRTHA^IPTO^^, Mass. 



