October 30, 1895.] 



Garden and Forest. 



433 



Lake Texcoco, which in turn overflowed and flooded the 

 valley. In June, 1629, the date of the last g^reat flood, the 

 city was covered with water to a depth of three feet, and 

 it remained in that state for five years. 



The regular fields were, of course, ruined whenever a 

 freshet traversed the valley, and necessity finally compelled 



the absence of an enzyme or organic ferment, which would 

 have been due to the presence of fungi or bacteria, this was 

 accepted as evidence of the absence of these organisms." 



Experiments were then made with the view to finding if 

 the spot was caused by atmospheric conditions. A young, 

 healthy plant of Habenaria Susannas was taken from a 

 tropical house and placed in a tempera- 

 ture of forty-one to forty-five degrees, 

 Fahrenheit, for twelve hours, and mi- 

 nute particles of ice were placed at in- 

 tervals on the uninjured epidermis of 

 the upper surface of the leaves. Twenty- 

 four hours later the points on the surface 

 of the leaves originally covered by par- 

 ticles of ice were pale in color, and 

 within four days every phase of the 

 disease was to' be seen. In fact, Mr. 

 Massee succeeded, within the time 

 stated, in producing a very clear and 

 bad case of spot in a plant which pre- 

 viously was perfectly healthy. This 

 result was abundantly corroborated in 

 the garden. A batch of plants of this 

 same species of Habenaria, which had 

 become drawn somewhat in a stove 

 during the exce-sively hot weather ex- 

 perienced in the early part of the sum- 

 mer, was removed into a cooler house. 

 Within a short time after their re- 

 moval a spell of cold weather was 

 experienced and the plants suffered a 

 check which resulted in their becoming 

 very badly affected by spot. In the 

 same house were some very healthy 

 plants of several species of Satyrium, 

 which, during the warm weather, grew 



Fig. 59. — Boats on La Viga Canal, loaded with vegetables and flowers. — See page 432 



the people to depend upon floating gardens for a supply of 

 produce at all seasons, and to prevent a famine. These 

 were moored in places where the rise and fail of the lake 

 waters would not affect them. During the period when 

 floods were looked for at any time, these floating patches 

 were very common, but when the city and valley were par- 

 tially protected by a gigantic canal in 1789 (commenced 

 in 1607*), by which the main overflow was carried off in 

 safety, they gradually disappeared, until at the present 

 time nothing but the pretty name and stationary plots sur- 

 rounded by water remains to perpetuate an ancient custom. 



Washington, D. C. CharlcS H. Coe. 



Foreign Correspondence. 



The "Spot" Disease of Orchids. 



IN the September number of the An7ials of Botany (Clar- 

 endon Press, Oxford), there is a paper by Mr. G. Massee, 

 of Kevv, on what is known by cultivators as Orchid spot. 

 The nature of this disease, the worst of all the enemies 

 against which the Orchid-grower has to contend, has been 

 a much-debated question among physiologists, fungolo- 

 gists and cultivators, some holding that it was fungoid and 

 infectious, others that it was caused by the punctures of 

 insects, and others that it was solely due to wrong treat- 

 ment, and was entirely under the control of the cultivator. 

 Mr. Massee's investigations go to prove that the last hypoth- 

 esis was the right one. He set out with the preconceived 

 idea that the disease was fungoid, and at first his researches 

 pointed in that direction. This theory, however, breaking 

 down, a search was made for bacteria, but with a like 

 result. " Failing to induce the disease in healthy plants 

 by inoculation with the expressed juice from diseased spots, 

 even when introduced under the epidermis, thus proving 



*The drainage canal, commenced by the Aztecs, has been greatly improved and 

 only recently finished by the Mexican Valley Drainage and' Canal Company, so 

 that all surplus water and the sewage ot the city is now completely carried otf. 



with exceptional vigor, but when the 

 spell of cold weather came they, too, 

 fell sick with spot. 



Mr. Massee found that a drop of cold water was equally 

 conducive to the formation of spot. He also found, after 

 numerous experiments, that the disease did not put in an 

 appearance unless the fall of temperature to which the 

 plant was subjected exceeded nine degrees, Fahrenheit, 

 below the average temperature in which the plant had pre- 

 viously grown. Tropical plants are much more liable to 

 become "spotted" than those grown in a lower tempera- 

 ture, and this was proved by Mr. Massee's experiments, 

 which again agrees with the experience of cultivators. The 

 only exception that occurs to me is the genus Masde- 

 vallia, the species of which are almost entirely grown cool, 

 but which are rarely free from spot. Still, experience goes 

 to prove that in their case excessive moisture, particularly 

 at the root, is most conducive to spot. The healthiest col- 

 lections are those where the plants are allowed far less 

 water than they are popularly supposed to require. 



The following extract from Mr. Massee's paper is impor- 

 tant as showing that spot generally attacks the younger 

 leaves at a time when they are in active growth : " Irregu- 

 larity in the appearance of the spot in different specimens 

 of the same species, even when conducted under precisely 

 similar conditions as to temperature, showed that some 

 other undetermined factor exercised an influence. After 

 repeated experiments this proved to be the relative amount 

 of moisture present in the plant. After a pseudo-bulb, with 

 its accompanying leaf, had been removed from a plant and 

 allowed to rema~in for three days in a dry place, it was 

 found impossible to produce spot by the method men- 

 tioned above, whereas with a similar specimen removed 

 from the same plant, and having the pseudo-bulb placed in 

 water at once, fully developed spot could be produced in 

 four days. Similar results were obtained when experi- 

 ments were made with entire plants ; those copiously 

 supplied with water at the root and grown in a high tem- 

 perature spotting readily ; whereas plants in a resting 



