1903. I*ETHYBRIDGE. — The Leof-spots oj Artivt fnaculatum. 151 



for say 24 hours. The sugar which will have diffused out 

 in this time could be titrated in the ordinary way, and its 

 amount compared with that diffused from an equal weight of 

 the green parts of the leaves treated similarly. 



Coming lastly to the function of the spots, Stahl has shown 

 by careful thermo-electric and other experiments that the 

 parts of plants coloured red by anthocyan become slightly 

 warmer than the merely green portions under similar con- 

 ditions of radiation. Plants with such coloured leaves are 

 developed at their best in the shade of tropical forests where, 

 the air being practically constantly saturated with moisture, 

 transpiration is difficult. The slight increase of temperature 

 at the red portions of the leaf, however, would allow of the 

 evaporation of water at these spots. 



In a normal green foliage leaf the s"tructure is that resulting 

 from the necessity of carrying on the two great functions, 

 transpiration and photosynthesis, by the tissues of the leaf as 

 a whole. In a red-spotted leaf there would seem to have been 

 developed a structure admitting of at any rate a partial division 

 of labour, the spotted portions being the better equipped for 

 promoting transpiration. The decreased thickness of the leaf 

 the poor development ofthe palisade parenchyma, the increase 

 in intercellular space, the increase of surface exposed (in the 

 depressed spots), as well as the presence of the colouring 

 matter itself, all suggest the possibility of increased transpira- 

 tion at the spots in the leaves of Arum maculatum. Whether 

 this is actually the case or not, experiment of course alone can 

 decide. 



Can it be that the Arum is gradually losing its spots ? One 

 finds the plant often in comparatively exposed situations, and 

 even in woods and hedges its leaves are well developed and 

 have been actively at work some ^tinie before the unfolding 

 leaves of the trees and shrubs produce any appreciable shade. 

 Does the plant in this island take time by the forelock and, 

 coming up early before there is much shade, dispense with 

 shade requisites, and can we infer from the rarity of the spotted 

 form in Ireland that we are far from being always under a cloud 

 in this country ? 



Roj^al College of Science, Dublin. 



3 2 



