50 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



circumstances are less favourable, and with severe fogs the foliage 

 of some Orchids and of certain other families of Monocotyledons 

 are affected. But even here the number of species which show 

 signs of severe suffering is small as compared with Dicotyledons. 

 It is amongst hothouse Dicotyledons that the bulk of the damage 

 to foliage is met with. 



On a preceding page I pointed out that the effect of reduced 

 light is to lower the tone of plants and to undermine their con- 

 stitution, so that they are less able to resist actual poison in the 

 air. It is these two circumstances of the environment, playing 

 as it were into each other's hands, that lead, in my opinion, to 

 the results observed. But Ferns — speaking generally — are ex- 

 ceptional in this respect. Ferns, as a class, may be termed shade 

 plants, and as such are not seriously affected by periods of partial 

 darkness. Other things being equal, the more greedy a plant is 

 of sunlight, the more will it suffer when its illumination is re- 

 duced. If, then, the constitution of the Fern is not seriously 

 undermined in dull and foggy weather, may it not perhaps on this 

 account be the better able to resist the poisonous gases ? May not 

 the apparently extreme delicacy of the Fern's organisation be more 

 than compensated for by its immunity from any serious lowering 

 of tone in partial darkness ? I put this solution of the question 

 forward with some reserve as I know I have not sufficient evidence 

 to prove it. It may reasonably be urged that, amongst Dicoty- 

 ledons, such as are shade plants should likewise resist injury. 

 Many undoubtedly do, but I am not as yet in a position to say 

 that this is anything like general. 



Monocotyledons are not so easily dealt with as Ferns, though, 

 taken all in all, cultivated Monocotyledons are shade-loving 

 when compared with Dicotyledons. One thing seems pretty 

 certain, and that is that Monocotyledons do not suffer in the 

 sin iic degree that Dicotyledons do from diminished illumination. 

 And there is good reason for believing some fundamental physio- 

 logical difference between the properties of the leaves of these 

 two classes of plants exists, though a satisfactory explanation of 

 this difference is not as vet forthcoming. The following extract 

 from Yines's "Physiology"* supports this assertion: "One of 

 the most striking features presented by plants which have been 

 grown in darkness is the smallness of the leaves. This is not a 

 * fc>. II. Vines, Lectures on the Physiology of Plants, p. 380. 



