550 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



to the list, but we should expect that they would fall under one or other of the 

 above-mentioned groups. 



Gall-structures have a peculiar significance for the section of the Natv/ral His- 

 tory of Plants which deals with the question of the origin of species, since they 

 show most distinctly how fundamental deviations from the original plan of con- 

 struction may occur in the adult condition of a portion of a plant. 



We must be careful to remember always that the immense variety of structures 

 which we call galls would not have existed except for the effect produced on the 

 plants by mites, leaf -lice, gnats, wasps, &c. The foliage of Rhododendron would not 

 have been rolled up, but quite flat, if gaU-mites had not been present; the branches 

 of Pistacia Lentiscus would have borne pinnate foliage with shining dark-green 

 leaflets and not fleshy-red pods if they had not been attacked by leaf -lice ; the leaf- 

 bud of Quercus pubescens would have developed into a long leafy shoot instead of 

 a body like a medlar if the gall-wasp Gynips polycera had not provoked the 

 change; the foliage of Veronica saamtilis would not have been lobed like a hand, 

 but would have had a slightly sinuous margin; and the upper leaves of Thymus 

 would have been green, spoon-shaped, and smooth on the surface instead of circular 

 and covered with white hairs, had no gall-mites settled on them. The flowers of 

 Rhododendron ferrugineum, Lychnis Viscaria, Veronica, Ga/rdamine, &c., would 

 not have "doubled", and the stamens would not have changed into petals, if they 

 had not been under the influence of gall-mites. 



Of course, the influence of the animals can only produce these effects on parts 

 of plants which are in an embryonic condition. Mature stems and leaves may 

 be eaten and destroyed by insects, but they can no longer be metamorphosed. But 

 the undifferentiated rudiments upon which the influence is effective are, so to speak, 

 formless. Leaves, stems, and fruits arise from tissue-masses having the form of 

 tubercles and cushions, and each tubercle or cushion originates from a few cells 

 which give no indication of what is to develop from them. Nevertheless, experi- 

 ence teaches us that the plan of construction for the plant-member proceeding from 

 these primitive forms is definitely laid down from the beginning for each species, 

 and the idea that the plan of construction is rooted in the specific constitution of 

 the protoplasm of the plant— i.e. in the cell or cells which form the primitive stage 

 or rudiment of the developing leaf, stem, &c., is confirmed. If an alteration in 

 this plan of construction is produced by these animals, it can only be by some 

 alteration of the specific constitution of the protoplasm. 



How the alteration is effected is just the puzzle which is at present occupying 

 the attention of naturalists. Once it was thought that the formation of galls was 

 the result of injuries caused in the growing tissue by the ovipositor or sucking 

 organ of insects, but recent investigations have not confirmed this view. The cells 

 actually injured by the insect in laying its eggs perish, and consequently lose the ' 

 power of metamorphosis or of producing modified daughter-cells. Cork, closing 

 over the wound, is always formed from the adjoining living tissue, but foi a long 



