EXTERNAL CAUSES OF GROWTH AND FORMATION. II 



323 



A'a' 



notice an apical elongation the gall proper and basally a number of unaltered 

 bud-scales. A longitudinal section through a young stage is shown at Fig 96, V. 

 In the centre lies a space containing the larva ; round it there is a layer of cells, 

 characterized by their large nuclei and abundant proteid 'and fatty contents. 

 This we may term the nutritive layer, for on it the larva feeds. The whole 

 of the space between the nutritive layer and the papillate epidermis is com- 

 posed of greatly thickened amyliferous cells which remain in the ripe gall. 

 Finally vascular bundles branch abundantly through the cortex of the gall from 

 below upwards. 



At the beginning of June the insects escape ; this time some are male, 

 some are female. The latter are like the leaf-wasps but are much smaller. 

 These insects were known by the name of Spathegaster taschenbergi before 

 their relation to Dryophanta /o/n was known, 

 and hence even yet the galls are spoken of 

 as ' Taschenberg galls '. The female ' spa- 

 thegasters' after fertilization betake them- 

 selves to the under side of immature leaves, 

 bore deeply into one of the larger veins with 

 their ovipositors and lay eggs in the canal so 

 formed. A gall then arises from the phloem 

 of the neighbouring vascular bundle which 

 soon bursts the cortex (Fig. 97, /), and in a 

 central space the young larva develops after 

 being released from the egg-shell (//). The 

 gall developing thus endogenously, like a 

 root, grows until it becomes an externally 

 visible sphere, fastened only by a short pe- 

 duncle to the inner edge of the leaf- vein. It 

 shows a differentiation of tissue (///) into 

 three concentric layers. The innermost 

 forms a nutritive layer for the larva, then 

 follows a sclerotic layer, and externally a 

 thick cortical region plentifully supplied with 

 vascular bundles. At this stage, however, 

 the gall is by no means fully formed. Later 

 on the sclerotic layer and the cortex increase 

 markedly, and solitary thin-walled cells, as 

 well as the thinnest-walled sclerotic cells, 

 grow greatly and become at the same time 

 filled with reserves. BEIJERINCK terms this 

 the secondary nutritive layer, since it, like 

 the primary layers, serves for the support of 

 the larva ; the largest cells are always to be found just where the larva feeds. 



Obviously the larva excretes something which acts as a stimulus to cell 

 growth. This stimulus may be either chemical or mechanical, and the question 

 comes to be what is the stimulus which induces the gall-formation as a whole. 

 First of all it is certain that the wound caused by the puncture made by the 

 insect need not be taken into account. Further, the movements of the larvae, 

 looked at as mechanical stimuli, cannot be made answerable for the formation 

 of the gall, since gall-formation commences while the larva is still completely 

 enclosed in its egg-shell. Some definite substance must diffuse out from the 

 larva which stimulates the cells to hypertrophy. In certain cases, e.g. in Ne- 

 matus-gafts on willows, according to BEIJERINCK (1888) the female during the 

 process of laying the egg excretes a substance which induces gall-formation, 

 so that small galls may develop without the development of a larva, but 



Y 2 



Fig. g6. Galls of Spathegaster taschenbergi. 



same after enclosure of tfie egg; gW ; 

 investing tissue; LK larva; AY, cavity, 



iv, 



starch layer; e p, epidermis. 



