THli ORCHID WORLD. 



39 



without disposing of the moisture, spottmg of 

 the leaves will surely result. 



Boxall, when collecting Vanda cc^^rulca in 

 the Southern Shan States, frequently had to 

 contend with extremely hot, humid days, yet 

 at night-time it was so cold that hoar frost 

 was formed on the ground and very often on 

 the plants themselves. He, however, always 

 noticed that, before the arrival of the cold and 

 frost, keen drying winds came and carried 

 away almost all the moisture in the atmo- 

 sphere ; the plants, therefore, nex er suffered 

 from the effect of a saturated atmosphere at 

 a low temperature. 



" Spot " on the leaves of Orchids may be 

 attributed to an unnatural or excessive amount 

 of atmospheric moisture when the tempera- 

 ture is low, and the plant is fully charged with 

 water. This spot disease of Orchids was in- 

 vestigated by Mr, George Massee, of Kew, 

 and the following experiments which he 

 carried out are recorded m the Annals of 

 Botany, Vol IX., No. XXXV., Sept., 1895. 



A young, healthy plant of Habenaria Sus- 

 annas, perfectly free from spot, and which up 

 to the date of the experiment had been grow- 

 ing in a house having a temperature ranging 

 between 75° and 80° F., was selected for 

 experiment. Minute particles of ice were 

 placed at intervals on the uninjured surface 

 of the leaves. The plant — along with the pot 

 in which it grew — was then placed in a sink 

 and covered with a bell-jar, and cold water 

 from a tap allowed to flow over the bell-jar for 

 twelve hours, during which time the tempera- 

 ture inside the jar ranged between 41° and 

 45° F. Twenty-four hours after the experi- 

 ment the places on the surface of the leaves 

 originally covered by particles of ice were pale 

 in colour, and on examination under the 

 microscope commencement of the disease 

 could be traced. The remaining spots were 

 examined at intervals, and within four days 

 every phase of the disease was observed, 

 agreeing in every respect with the features 

 already known. 



A second experiment, in which drops of 

 water at a temperature of 45° F. were placed 

 on the leaves instead of particles of ice, gave 



similar results, the spotting only occurring on 

 the portions of the leaf which had been 

 moistened. Numerous additional experi- 

 ments showed that " spot " could not be 

 produced by a fall of less than g° F. from the 

 average temperature in which the plant had 

 been growing. It was also noticed that plants 

 whrch had jjreviously been grown in a high 

 temperature became diseased at a much 

 smaller reduction of temperature than plants 

 previously accustomed to a comparatively low 

 temperature. 



It was found impossible to produce " spot " 

 on plants which had remained in a dry place 

 for two or three days, or in those scantily 

 supplied with water and kept in a low tem- 

 perature. Mr. Massee's summary is briefly : 

 That the Orchid disease known as " spot " is 

 of non-parasitic origin ; the initial cause being 

 the presence of minute drops of water on the 

 surface of the leaves at a time when the tem- 

 perature is exceptionally low, and the roots 

 copiously supplied with water. " Spot " in the 

 broadest sense of the term is, in the case of 

 cultivated Orchids, mainly, if not entirely, 

 caused by the three following conditions : — 

 (i) too high a temperature ; (2) too much 

 water, and not sufficient air in contact with the 

 roots ; (3) watering or spraying with a falling 

 instead of a rising temperature. 



In the early autumn and spring months the 

 sun is frequently of sufficient power to burn 

 the leaves of Orchids in a house when the 

 cold winds will not allow of much ventilation 

 being used. If the blinds are used at such 

 times of the year excessive shade would be 

 produced, and much of the sun's warmth would 

 be lost. By closing the ventilators and 

 thoroughly damping the floor and other ex- 

 posed positions, a considerable amount of 

 moisture wiill be condensed on the inner side 

 of the glass, and this shading will prove quite 

 sufficient to prevent the burning rays of the 

 sun from injuriously affecting the plants. 

 When, after a few hours, the sun has lost its 

 power, the ventilators may be slightly opened, 

 and any excess of moisture thereby disposed 

 of. 



G. W. 



