212 THE BIOLOGY OF FLOWERING PLANTS 



however, consider the case of the succulents, which we have 

 seen show very high temperatures. MacDougal's values for 

 the transpiration of the cactuses are very irregular, but we 

 may take two widely different examples. During May (arid 

 month) an Opuntia weighing 140 grm. lost water at the 

 rate of 0*337 g^^- P^^ ^^Y- During March an Echino- 

 cactus of 18 kg. lost 30 grm. per day. For the Opuntia 

 this represents 200 grm.-calories, and for the Echinocactus 

 17,700. Even if we assume that this energy is all lost in 

 the hours of daylight, and this is certainly not the case 

 as has been shown by E. B. Shreve (19 16), we find that 

 the Opuntia loses in the course of the day about the same 

 amount of energy as a piece of sunflower leaf weighing 

 0'025 g^n^- with an area of 07 sq. cm. The energy loss 

 of the Echinocactus is equalled by i"9 grm. of sunflower 

 leaf measuring 65 sq. cm. The figures do not allow us to 

 make a comparison for areas, but they are sufficient to show 

 that the desert cactuses can be cooled by transpiration to 

 an inappreciable extent only. 



In halophytic succulents difl^erent conditions prevail, 

 for, as Delf (191 1) has shown, the transpiration may, for 

 equal areas, be twice as vigorous as in an ordinary mesophyte ; 

 here the lowering of temperature must be correspondingly 

 important. 



An investigation by Bergen (1904) gives some idea of the 

 conditions in sclerophyllous trees. He measured the 

 transpiration of sun and shade leaves of Olea europeea, 

 Quercus Ilex, Pistacia Lentiscus, and Rhamnus Alaternus. 

 These leaves do not differ in the same way as do the sun and 

 shade leaves of deciduous trees. The sun leaves are 

 smaller, usually paler, and, except in Pistacia, do not show 

 more strongly developed pallisade ; their stomata are more 

 numerous. With few exceptions the sun leaf transpired 

 more rapidly than the shade leaf when both were insolated, 

 or both shaded. The difference may be due to the different 

 numbers of stomata. The sun leaf would seem to be more 

 efficiently cooled. 



The case of the Cactaceae is at first sight a difficult one. 



