548 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



shoots have developed from the axils of its leaves. These lateral shoots again 

 develop lateral axes in the axils of their leaves, and so on to the third, fourth, and 

 fifth degree. Thus, in the course of a month, shoots have unfolded, which, except 

 for the influence of the gall-mites, would not have followed one another for three, 

 four, five, or even six years, and therefore these galls afford us another instance 

 of what has been termed "prolepsis" or precocious development of structures 

 which would not yet arise. Of course all the axes of these shoots are dwarfed 

 and the leaves which clothe them are diminished in size. The shortening and 

 diminishing increase gradually, so that the axes and leaves of the fourth and 

 fifth degree are much smaller than those of the second and third. The last 

 lateral shoots remain bud-like, and their small scaly leaves fold over one another 

 like the bracts in the involucre of a Composite. The "witches' brooms" which 

 are caused by gall-mites on Lilac (Syringa vulgaris) and Privet (Ligustrum vul- 

 gare) bushes are similar in nature to these closed galls on the Willows. Frequently 

 the metamorphosis of the leaves on the axes of the third, fourth, and fifth degree 

 includes those of the floral region, and such cases form to some extent a bond of 

 union between cluster-galls on foliage and on floral regions, respectively. 



One of the most remarkable changes exhibited by the gall-structures just men- 

 tioned, viz. the abbreviation of the axis, is of course not to be noted in cluster- 

 galls in the floral region. The part of the axis which forms the floral receptacle 

 does not grow into an elongated shoot, but always remains short, and the floral - 

 leaves it bears stand close to one another, forming whorls in whose niches and 

 recesses numerous small animals can hide. But these animals effect other very 

 marked alterations by their stimulus. In some flowers, instead of the normal 

 red, blue, white, or yellow petals, green leaflets appear which resemble foliage- 

 leaves in character, and then we say that the flowers have become "green" or 

 " leafy ". In other plants the stamens are transformed into petals, and the flowers 

 are said to be " double ". Finally, it may happen that the carpels which are usually 

 united together to form a syncarpous ovary stand on the receptacle as distinct 

 structures, and that to a certain extent their union has been dissolved. In these 

 cases we speak of " antholysis " (cf. p. 80). The influence of gall-mites also produces 

 metamorphosed flowers which may be both green and double, and in which the 

 pistil may have separated into its individual carpels. 



The best flowers for observing these metamorphoses in all imaginable degrees 

 are the small-flowered species of the Chickweed genus (Cerastium macrocarpum, 

 triviale, &c.), several Caryophyllaceae (Lychnis Viscaria, Saponaria officinalis, 

 fec.), Cruciferae (Cardamine uliginosa, Camelina sativa, Lepidium Draba\ 

 Gentians (Gentiana acaulis, rhcetica), Speedwells (Veronica officinalis, saxatilis) 

 and Milfoils (Achillea Millefolium, nana). In Speedwells the petals come to 

 resemble leaves. The bunches, rosettes, and balls of small green leaves replacing 

 the flowers are set close together on the rachis of the inflorescence and form green 

 racemes and tufts, sometimes even small witches' brooms. In Veronica saxatilis 

 the rachis of the raceme, the pedicels, and the bracts are covered with hairs, which 



