DIV. II PHYSIOLOGY 295 



leaves appear. Another peculiar example of abnormal growths is afforded by the 

 GALLS or CECIDIA produced on plants by Fungi, or more frequently by insects, 

 worms, and arthropods. The effect of these formations on the normal development 

 of the tissues of a plant is more or less disturbing, according to their position, 

 whether it be in the embryonic substance of the growing point, in the tissues still 

 in coarse of differentiation, or finally in those already developed. Galls which are 

 products of abnormal tissue formation are termed HISTOID, while ORGANOID galls 

 depend on the transformation or new formation of members of the plant body. 

 The latter are especially instructive. The larvae of Cecidomyia rosaria live in the 

 growing points of Willow stems, and occasion a malformation of the whole shoot 

 by the production of galls, known as " willow -roses," which are composed of 

 modified leaves and axes. Flies (Diptera) often deposit their eggs in the tissues of 

 partially-developed leaves, in consequence of which the leaves become, according 

 to their age when attacked, more or less swollen and twisted. After the leaves of 

 the oak have attained their full growth they are often stung by a gall-wasp of the 

 genus Cynips. The poison introduced by the sting, and also by the larvae hatched 

 from the eggs deposited at the same time, occasions at first only a local swelling of 

 the leaf tissue, which finally, however, results in the formation of yellow or red 

 spherical galls on the lateral ribs on the under side of the leaf. 



Symbionts, i.e. associated, mutually-beneficial organisms, neither 

 of which can be regarded as the host, may influence one another 

 formatively. This is seen, for example, in Lichens. 



It is probable that chemical substances play an important part in 

 the influences exerted by one organism on another. It is true that 

 only in rare cases have deformations resembling galls been brought 

 about by the action of dead substances extracted from the normal 

 inhabitant of the gall. Parasites which do not give rise to galls 

 probably act on the host plant by poisonous substances. On the other 

 hand, the host plant by forming anti-bodies may injure the parasite or 

 prevent its entrance. Thus HEINRICHER has shown that some kinds 

 of pear-tree are readily infected by the mistletoe and others only with 

 difficulty ; he has also shown that probably one infection by the 

 parasite renders the host more resistant to artificial infections. There 

 are thus PHENOMENA OF IMMUNITY in the vegetable kingdom, though 

 they have not been nearly so thoroughly investigated as in the case of 

 animals ( 60a ). 



7. Pupposiveness of the Reactions to External Factors. It has 

 been seen that the form and structure of the plant is influenced in a 

 regular fashion by many external factors. While some of the resulting 

 changes are without importance to the plant or, as in the case of galls, 

 are only of use to the organism causing the change, the majority of 

 reactions to external stimuli are remarkably purposive, i.e. they are 

 of use to the plant. Examples are afforded by the elongation in 

 etiolation, the characteristic development of amphibious plants in 

 water and on land, the increase of protections against transpiration 

 with the greater dryness of the atmosphere, etc. ; these purposive 

 reactions are termed ADAPTATIONS. How it comes about that the 



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