172 



THE BOOK OF CHOICE FERNS. 



from the juice and distributed in the form of steam all over the house, 

 which, when of very large dimensions, may require two, three, or even more, 

 similar pots and bricks. By this process not only does the nicotine, through 

 its purity, remain harmless to the plants, but it also reaches green and white 

 flies as well as thrips. Thrips have a very clever way of instinctively letting 

 themselves down to the ground as soon as the smoke approaches them, and 

 in that way partially, and sometimes even totally, avoid its effects. Another 

 point in favour of tobacco -vapour lies in the fact that there can be no flaring 

 up, as is frequently the case with tobacco-paper or with tobacco-rags, when 

 the effects are really disastrous. 



Like tobacco-smoke, tobacco-steam is destructive only to breathing insects, 

 and then only when these have no means of escape ; but it has no effect either 



on mealy bug (Fig. 14), which must be removed 

 by hand — all the insecticides which up to the 

 present have been found capable of destroying it 

 being at the same time equally injurious to 

 Ferns — or on scale (Figs. 15 and 16), which must 

 ng. 14. Mealy Bug also be washed off or picked by hand. The latter 



(magnified and nat. size). m Sec t, which has all the appearance of a small 



protuberance of either brown or whitish colour, 

 although apparently remaining stationary, multiplies, and, notwithstanding the 

 apparent absence of legs or of any other means of locomotion, spreads rapidly, 

 mainly through the agency of ants, which derive some nourishment from its 

 exudations. Accordingly, the ants — remarkable alike for their activity and 

 their foresight — form "plantations" of scales, much in the same way as we 

 form orchards, oyster-beds, &c, by carrying them from one plant to another 

 (a performance which has many times been watched with keen interest by 

 observers), and setting the mother scale so that it may easily adhere to the 

 stem or the under-part of the frond, where a young colony surrounding the 

 old scale shortly makes its appearance, and, unless its progress be checked at 

 the outset, soon spreads over the whole plant on which it lives. This mode of 

 transport by ants explains the spreading of scales from one plant to another 

 of the same kind situated at a considerable distance, and for which it is 

 difficult, if not even impossible, to account in any other way. The scales are 

 very difficult to remove, on account of the inefficiency of known insecticides, 



