MORPHOLOGY 



taken advantage of by horticulturists to bring about certain wished 

 for results. Among external causes of variations the influence 

 exercised by parasites upon the development • of the whole plant is 

 particularly striking. Euphorbia Cyparissias, when attacked by a rust 

 fungus (Aecidium Eupharbiae), becomes sterile, remains unbranched, 

 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 are the galls or CECIDIA produced on plants by Fungi, 

 or more frequently by insects. 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, or in the tissues still in 

 course of differentiation, or finally in those already developed. The 

 larvie of Cecidomyia rosaria live in the growing points of willow stems, 

 and occasion a malformation of the whole stem by the production of 

 galls known as "willow -roses." Flies (Diptera) often deposit their 

 eggs in the tissues of partially developed leaves, in consequence 

 of which the leaves become 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, occasion 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 materially differ 

 from one another according to the nature and cause of their forma- 

 tion, it is generally possible to determine the insect or Fungus by 

 which they were induced. As an explanation of malformations which 

 originate in the plants themselves, some exciting cause must be pre- 

 sumed which turns the processes of development from their usual 

 course. The earlier such an influence makes itself felt in the rudi- 

 ments of organs the more severe is its effect upon their development. 

 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. 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 stem may be de- 

 veloped, 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 



