266 THE RATE OF GROWTH [ch. 



of many kinds, well-defined and widely different, each caused to 

 grow out of the tissues of the plant by a chemical stimulus contri- 

 buted by the insect, in very minute amount; and the insects are 

 so much alike that the galls are easier to distinguish than the flies. 

 The same insect may produce the same gall on different plants, 

 for instance on several species of willow ; or sometimes on different 

 parts, or tissues, of the same plant. Small pieces of a dead larva 

 have been used to infect a plant, and a gall of the usual kind has 

 resulted. Beyerinck killed the eggs with a hot wire as soon as 

 they were deposited in the tree, yet the galls grew as usual. Here, 

 as Needham has lately pointed out, is a great field for reflection 

 and future experiment. The minute drop of fluid exuded by the 

 insect has marvellous properties. It is not only a stimulant of 

 growth, like any ordinary auxin or hormone; it causes the growth 

 of a peculiar tissue, and shapes it into a new and specific form*. 



Among other illustrations (which are plentiful) of the subtle 

 influence of some substance upon growth, we have, for instance, 

 the growth of the placental decidua, which Loeb shewed to be due 

 to a substance given off by the corpus luteum, lending to the uterine 

 tissues an enhanced capacity for growth, to be called into action by 

 contact with the ovum or even of a foreign body. Various sexual 

 characters, such as the plumage, comb and spurs of the cock, arise 

 in hke manner in response to an internal secretion or "male 

 hormone " ; and when castration removes the source of the secretion, 

 well-known morphological changes take place. When a converse 

 change takes place the female acquires, in greater or less degree, 

 characters which are proper to the male: as in those extreme cases, 

 known from time immemorial, when an old and barren hen assumes 

 the plumage of the cockf. 



The mane of the lion, the antlers of the stag, the tail of the peacock, 

 are all examples of intensifled differential growth, or localised and 



* Joseph Needham, Aspects nouveaux de la chimie et de la biologic de la croia- 

 sance organisee. Folia Morphologica, Warszawa, viii, p. 32, 1938. On galls, see 

 {int. al.) Cobbold, Ross und Hedicke, Die Pflanzengallen, Jena, 1927; etc. And 

 on their "raorphogenic stimulus", cf. Herbst, Biolog. Cblt., 1894-5, passim. 



t The hen which assumed the voice and plumage of the male was a portent or 

 omen — gallina cecinit. The first scientific account was John Hunter's celebrated 

 Account of an extraordinary pheasant, and Of the appearance of the change 

 of sex in Lady Tynte's peahen, Phil. Trans, lxx, pp. 527, 534, 1780. 



