No. XV.B.] APPENDIX. 229 



flourishing, but so soon as the insects were developed, and preyed upon 

 its juices, it became deteriorated, and manifested unequivocal symptoms 

 of disease. To-day I placed some living aphides upon the leaves of the 

 healthy potato-plant on the table, upon which they are actively feeding, 

 and, if means are not adopted to stop their ravages on this plant, it will 

 become diseased and die in consequence. Then from these examples we 

 may infer that aphides appear first on healthy plants, and, therefore, live 

 on healthy vegetable matter. 



In the next place, in order to understand the process of this disease, 

 we should know the part of the plant that aphides exist on, and which 

 must be either solid or fluid. To determine this point an examination 

 of their apparatus for feeding becomes necessary. This is extremely 

 delicate. Situated on the under-side of the head is the rostrum, which 

 is about one-fourth as long as the whole body, and contains a fine 

 instrument for piercing the leaf and the walls of its Jcells : this is found 

 to consist of three fine setae or delicate piercers, one of which answers 

 to the tongue and the others to the jaws of the insect. These are 

 very beautiful objects for examination with the microscope. If an aphis 

 be examined by means of a magnifying- glass, whilst attached, it will be 

 seen to have a sort of proboscis applied to the leaf, and if touched it will 

 be found to adhere pretty firmly by this, which constitutes its suctorial 

 apparatus, and it requires some seconds to disengage itself from its posi- 

 tion. This apparatus, by which it is so closely attached, is that by which 

 it is enabled to pierce the leaf, break up its cells, and suck the vital fluid 

 as it passes through the leaf to be rendered fit for the nourishment of 

 the plant. 



From these facts, then, we deduce that aphides suck the juices of 

 plants, which is analogous to the blood of animals, and, therefore, the sap 

 is impaired in its qualities in that vital organ the leaf, whence its power 

 of fulfilling its various functions is either weakened or destroyed. If the 

 sap is taken away, its functions cannot be performed ; and if its nutritive 

 properties are weakened, it cannot properly nourish the plant, which in 

 consequence becomes debilitated, and an imperfect vegetable tissue is 

 formed, that is in itself very prone to die, of which I have had numerous 

 examples in large tulips, crocuses, mangold- wurzel, potatoes, &c. Death of 

 a plant may be either local, i.e. confined to the spot where the aphis 

 penetrates the leaf or leaflet, as is shown by the spots seen on the various 

 kinds of leaf upon the table, as of the turnip, potato, horse-radish, &c. ; 

 or one single leaf, or a certain number of leaves or leaflets, may die with- 

 out the disease passing further into the plant. One portion of the stem 

 may perish and cut off the supply of nutriment from other leaflets, which 

 will die from this secondary cause, or the whole of the stem may be cut 

 down, and thus the due supply of properly elaborated sap being prevented, 

 the plant must perish. The plant may also begin to perish at the extreme 

 ends of the rootlets, the ascent of crude sap for elaboration being rendered 

 impossible. Any portion of the stem may also first manifest the disease ; 

 but most frequently the malady first shows itself at the part which is 

 technically and vulgarly called " the collar " of the plant (being the inter- 

 mediate part between the ascending and descending axes). This is very 

 frequent, especially in the potato and turnip. Then we find that partial 

 death following the attacks of aphides may be only local at the part where 



