166 BOTANY PART I 



normal development of the tissues of a plant is more or less dis- 

 turbing, 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 developed. The larvae of 

 Ceddomyia 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 round, yellow, or red galls on the lateral 

 ribs on the under side of the leaf. As galls differ 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 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 point of a root 

 may continue its further development as a stem. Leaves, even when 

 somewhat advanced in growth, may under changed conditions vary 

 their usual character, particularly within the limits of their possible 

 metamorphosis ; for example, the staminal and carpellary leaves of a 

 flower may thus become transformed into additional perianth leaves. 

 The later the rudiments of an organ are acted upon by a disturbing 

 influence, so much the less far-reaching are the modifications which 

 are produced ; thus intermediate forms between two organs may 

 be produced which correspond more or less closely to one or the 

 other of them. Neither the abnormal interchangeability, at times 

 manifested between morphologically different members, nor the 

 development of intermediate forms between them, can be considered 

 as proof of their phylogenetic connection. MALFORMATIONS ARE, 



ACCORDINGLY, TO BE ACCEPTED AS EVIDENCE IN MORPHOLOGICAL 



QUESTIONS ONLY IN THE RAREST CASES. When, as is only exception- 

 ally the case, they represent reversions, this value may be attached 

 to them. 



