SCIENCE-GOSSIP. 



GALL FORMATION. 



By Sophia Armitt. 



""PHE birds know better how to find the life that 

 is inside galls than do human beings. In 

 November and December they are searching among 

 fallen oak-leaves for the cherry-galls, and opening 

 them for the fat grubs that lie therein. An observer 

 who is interested in the habits of birds, and had 

 been watching them in the woods in December, 

 1893, brought in a lot of these cherry-galls and 

 placed them on moss inside a Dresden china cup 

 in the family sitting-room, to see what would 

 come of them. In the course of a few days, quiet 

 readers were frequently disturbed by the settling of 

 peculiar flies upon them in a markedly unpleasant 

 manner, causing involuntary and spasmodic starts. 

 Upon investigation it was found that the gall-flies 

 were emerging from the galls, and the bird observer 

 was requested to remove those galls to a different 

 place. This circumstance was calculated to arouse 

 curiosity. Were gall-flies really maturing and 

 emerging in winter ? If so, how would they get 

 along till the summer came and there were new oak- 

 leaves for them to put their eggs in ? 



Dr. Adler's book, reviewed in your last volume, 

 page 88, entitled, " Alternating Generations : A 

 Study of Oak-Galls and Gall-Flies, " solved these 

 questions. These flies (Dryophanta scutillaris) do 

 emerge, in any case, in winter from the cherry-gall. 

 It may be in nature they appear in January 

 or in February, but always after a frost, for 

 a thaw destroys the gall which is their home. 

 They are in this generation of only one sex, 

 and they live only a few days. These flies 

 search for little adventitious buds on the stem 

 of the oak-tree wherein they place their eggs. 

 In April the leaves from the buds pricked by the 

 flies produce new galls that are quite different 

 from the cherry-galls from which the flies emerged. 

 These galls are dark violet and velvety, and are 

 known as those of SpathegasUr taschenbergi. In 

 May and June the perfect flies of this new genera- 

 tion leave their galls. They are half the size of 

 the mother or winter-fly, and of two sexes. In a 

 few days the females begin searching for the 

 youngest and tenderest leaves, to prick the underside 

 of the veins, and place there their eggs. In each 

 pricked spot, when the egg hatches out as a grub, 

 will begin to grow a new cherry-gall, exactly like 

 the one in which the grandmother passed the 

 months which ended in the few days only of open- 

 air existence. 



The life story of the spangle-gall (Neuroterus 

 Uuticularis) varies from this. Everyone knows the 

 pretty spangles beneath the oak-leaves in July and 

 onwards. They fall in autumn on the leaves, but 



the life inside does not die with the leaf, it lives on 

 through the winter, and the fly comes out in April 

 or May. The gall-fly immediately begins to examine 

 buds carefully with its antennae ; when satisfied 

 with a suitable one, it pushes its ovipositor deep 

 therein, a long and difficult business, and lays one 

 egg. When the bud expands, a small round sappy 

 gall is seen either under a leaf or on a male flower- 

 catkin. This is the currant-gall (Spathegaster 

 baccarum), smaller when on the flower than when on 

 the leaf. From these the flies emerge in early 

 June, male and female this time. The young, 

 tender leaves are then sought for, and inside their 

 under surfaces eggs are placed from which spangle- 

 galls will form, serving as a home for their tiny 

 inmates, through summer and winter, till the next 

 year's new growing-time. 



Much of Dr. Herman Adler's interesting book 

 treats of the insects. There are minute descrip- 

 tions of their forms and stages of life history. I 

 have drawn the purely botanical parts together in 

 the following paragraphs. 



Galls occur on buds, leaves, flowers, bark or 

 root ; but wherever they are, they originate always 

 from the same parent tissue, from the formative 

 cells that are called the cambium ring. A layer of 

 this tissue extends through every plant from the 

 finest root fibres to the most distant leaves. All 

 vegetable life springs from the cambium layer ; its 

 cells are the theatre of actual metabolism, and yet 

 they are not differentiated into a stable tissue. It 

 is from these cells that all gall-formation proceeds. 

 When a gall-fly pierces the cambium layer and 

 deposits an egg there, gall-formation does not 

 certainly follow, it only begins when the larva 

 emerges from the egg. 



In this statement Dr. Adler differs from Sir 

 John Lubbock and others, and he limits it to the 

 action of oak gall-flies, having observed that flies 

 producing willow galls pour into the wound a 

 secretion which causes new cell-formation in the 

 course of a few hours. On the oak-tree, procedure 

 is different ; it is only when the larva breaks 

 through the egg-case and touches the surrounding 

 cells with its tiny mandibles that rapid cell-growth 

 is set up. Once begun, however, it goes on so 

 quickly that while one end of the larva is still in 

 its egg-case, a wall-like mass of cells has risen up 

 in front of it. This rapid cell-growth is due to the 

 irritation of the biting grub upon the highly 

 formative cells of the cambium, which possess 

 every condition for growth. 



One gall-fly (Trigonaspis cntstalis) pricks the 

 leaves in May ; it drives its ovipositor into the vein 



