124 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



DISEASE OF THE LEAVES OF CALANTHES. 

 By John Bidgood, B.Sc, F.L.S. 



During recent years there have been complaints from all parts that the 

 leaves of Calanthe plants grown in our Orchid houses have almost 

 universally been disfigured by broad areas and smaller patches of black 

 and dead tissue. This local death is not confined to the leaves, for the 

 flower-stems and bracts as well as the pseudo-bulbs show similar areas. 

 The consequence is that, long before their period of usefulness should be 

 over, the leaves are useless to the plant, while the flower-spikes and 

 pseudo-bulbs are small, and, like the leaves, unsightly. This more 

 particularly applies to the various hybrid Calanthes, the worst of all 

 being those which are most inbred. 



Except where otherwise stated, what follows here is more particularly 

 applicable to the hybrid section, to which my observations have been 

 almost entirely confined. 



The cause of the spotting is 'not easy to find, and is even now, after 

 three years' inquiry, observation, and experiment, mainly conjectural. 

 It is certainly not always produced by a fall of temperature in the gorged 

 leaf, as described by Massee in the " Annals of Botany,* September 1895. 

 He there shows that dead spots are produced on the leaves of certain 

 Orchids by the simultaneous operation of the three following conditions : 

 (1) too high a temperature ; (2) too much water and not enough air in 

 contact with the roots ; (3) watering or spraying with a falling instead of 

 a rising temperature. By experiment it was shown that drops of water 

 on the leaves always caused " spot " when the temperature fell excep- 

 tionally low and the roots at the same time were well supplied with 

 water. 



In the case of Calanthes, I am able to say that over and over again 

 I have kncwn plants to have become badly spotted when and where 

 drops of water cculd not have reached the leaves under any circumstances. 

 The "spot "in question is certainly of non -parasitic origin. Numerous 

 examinations and experiments have been made, but no fungoid or 

 bacterial parasite has ever been detected. Injuries by animals are also 

 not in question. 



The leaf of Calanthe shows an extreme type of hygrophilous structure. 

 It is large and broad, and its broadest part is well spread out horizontally. 

 It is very thin, consisting rarely cf more than three layers of spongy 

 parenchymatous cells, with abundant and large intercellular spaces, 

 covered by an upper and lower epidermis. The epidermal cells are small 

 and furnished with a very thin cuticle on each exposed surface. The 

 stomata are flush with the surface of the leaf. They are numerous on 

 the upper surface, but according to my observations on C. vestita, 

 C. x Veitchii, C. v. Regnierii, and many others, there are none on the 

 lower surface. There is a scanty supply of simple hairs on the upper surface 



