FORMATION OF GALLS 197 



bination of the possibilities of the plant, the peculiarities which are 

 combined remain the same, like the pieces which furnish the changing 

 pictures in a kaleidoscope. Formations intermediate between two organs 

 arise in this way very often, and Peyritsch has observed in Valeriana 

 intermediate formations between bracts and pappus-bristles and between 

 bracts and petals taking the place of the ordinary bracts. 



Seeing then that in these malformations nothing new appears, but 

 there is only a different combination of existing primordia, we must be 

 on our guard against assigning to malformations the phylogenetic signifi- 

 cance which is often assigned to them 1 . Granted that many characters 

 which exist in the plant as latent primordia ~ appear in increased pro- 

 portion when malformations develop, yet we have to deal with only 

 the unfolding of existences in a somewhat enfeebled condition, not with 

 a change of the whole formation of organs as is the case when, for example, 

 the ovules appear as leaflets in flowers which exhibit phyllody. In this 

 latter case the characteristic of the ovule, namely, the embryo-sac, 

 has disappeared. But when the bracts of an inflorescence of Cruciferae 

 develop one may upon comparative grounds always designate them 

 a reversion. In this way one should interpret the facts observed by 

 Treub 3 and others, that in the gall-formations of the capitulum of Hiera- 

 cium umbellatum every transition from normal pappus up to the occurrence 

 of five separate green leaves provided with vascular bundles is to be 

 found between the involucre and the middle of the capitulum where the 

 gall-apple sits. It is however only comparison with allied forms which 

 can lead us to an interpretation of this kind, for I am convinced that 

 there are latent primordia which were never developed in ancestors and 

 which therefore have no phylogenetic significance. Thus A. Braun and 

 Strasburger 4 have observed in Selaginella pentagona remarkable gall- 

 formations which are externally like bulbils. These have six rows of 

 leaves which are all constructed alike, whilst the leaves in all other known 

 species of Selaginella, except a few isophyllous forms, are arranged in 

 obliquely crossing pairs and in each pair the leaves are of unequal size. 

 These gall-shoots are inhabited by dipterous larvae, and the phyllotaxy 

 they exhibit is seen nowhere else in Selaginella. They grow by means 



of mites, are serviceable to the parasite and diverge altogether in structure from the normal hairs of 

 the plants on which they occur, 



1 See the first paragraph of this Section. 



2 Neottia nidus-avis, the well-known saprophytic orchid, has no green foliage-leaves, but 

 occasionally one may appear. Its primordium must have been latent. It may be assumed that 

 there is here an inheritance from ancestors furnished with foliage-leaves, but at the same time it is 

 allowable to ask is there any reason why there should not be latent primordia which are not 

 vestiges of an earlier development ? 



3 Treub, Notice sur 1'aigrette des Compose'es, in Arch. Ne'erl. viii. 



1 Strasburger, Die Bulbillen und Pseudobtilbillen der Selaginella, in Botan. Zeitung, 1873, p. 105. 



