154 



PRINCIPLES OP PLANT-TERATOLOGY. 



later, the bract subtending the seminiferous scale in 

 the female cone may become leafy. 



The most reduced and modified form of bracts to be 

 seen in the vegetable kingdom is that in the horse-tail 

 (Equisetum) in the form of the £t collar" subtending the 

 inflorescence. In E. maximum and E. arvense this 

 structure has been seen resolved into a whorl of 

 ordinary leaves united into a sheath at the base. 



All the phenomena just mentioned under this heading 

 are to be regarded as cases of reversion to that which 

 represents more or less perfectly the ancestral condition 

 of the organ concerned, viz., a foliage-leaf; concomi- 

 tantly with other parts of the reproductive region, the 

 bracts have, in the course of evolution, undergone 

 simplification. In Equisetum the normal leaves have 

 undergone excessive reduction as compared with those 

 of their ancestors ; hence, if still further degeneration 

 is to occur in order to give rise to the bracts or involucre 

 of the cone, we could hardly expect any better-developed 

 structure than the "collar" to result. In theCalamariea3 

 of the Coal Period the foliage-leaves were larger and 

 also free, not being united into a sheath, and the bracts 

 were of equal development. 



In all the above cases the change consists in a more 

 complete development of the foliar organ involved, 

 viz., the superadding of a blade or lamina to the 

 modified stalk or petiole of which alone the majority 

 of bracts may be supposed to consist. 



2. Heappearance of Bracts or Foliage-leaves. — 

 It is a very exceptional thing for a shoot to be wholly 

 devoid of foliar organs for any considerable portion 

 of its length. Scapes afford such an instance, and the 

 inflorescence of most Cruciferae. In many Cruciferge it 

 it is by no means rare for the bracts, which for some 

 unknown reason have been lost, to reappear, as in 

 the wallflower (Gheiranthus Gheiri), stock (Matthiola), 

 and rock-cress (Arabis) ; such cases are reversions to 

 the normal state exhibited by several genera, e. g. 

 Porpyhrocodon, Dipteri/gimn, and Selenia, in several 



