170 BOTANY part i 



Hugo de Vries (^) and regarded by him as alone of value in the 

 formation of species, in contrast to the ordinary variations which simply 

 oscillate between plus and minus. Abrupt and striking deviations of 

 the individual from the type of the species are called monstrosities. 

 When these induce a disturbance of function a diseased condition 

 results. The causes of monstrosities may lie in the organism itself ; 

 in other cases they can be shown to stand in relation to external 

 causes. Among EXTERNAL causes of variations the influence 

 exercised by parasites upon the development of the portions of the 

 plant is particularly striking. Eupharhia Cyparissias, when attacked 

 by a rust fungus (Aecidium Euphorbwe), becomes sterile, remains un- 

 branched, has shorter and broader leaves, and in its whole appearance 

 is so changed as scarcely to be recognisable. Plant lice sometimes 

 cause a flower to turn green, so that instead of floral leaves green 

 foliage-like 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, Avorms, 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 course of differentiation, or finally in those already de- 

 veloped. The larvae of Cecidomia 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," Avhich 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 round yellow or red galls on the lateral ribs on 

 the under side of the leaf. As galls difler materially from one 

 another according to the nature and cause of their formation, it is 

 generally possible to determine the particular exciting cause to which 

 they are due. 



In the monstrosities resulting from internal causes it is also the 

 case that the earlier they commence the more extreme do they tend to 

 be. When the embryonic substance of the growing point is affected 

 by such an influence altogether unexpected modifications of the usual 

 order of growth may result. In other cases intermediate structures 

 of more or less monstrous form result. As the embryonic substance 

 of the growing point is of itself capable of producing all such forms 

 as are peculiar to the species, instead of a flower a vegetative shoot 

 may be developed, or the growing plant of a root may continue 



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