594 LECTURE XXI. 



the lateral leaflets of Desmodium, or the nutations of growing 

 organs, yet in some cases the advantage is clear. This is 

 notably the case with regard to the circumnutation of tendrils 

 and of the stems of twining plants ; clearly the travelling 

 of these organs over a considerable area must largely in- 

 crease the chances of their coming into contact with a support. 

 The same may be said with regard to the movements of 

 those organs which are irritable to touch. It is not clear 

 what advantage the power of movement on mechanical stimu- 

 lation of the leaves of Mimosa brings to the plant ; but it is 

 clear in the case of irritable stamens : when an insect visits 

 a flower with irritable stamens it causes them to move, and 

 the result of their movement is a discharge of pollen some 

 of which is conveyed by the insect to another flower of the 

 same kind, and thus cross-fertilisation is ensured. 



It is impossible, within the limits of these lectures, to 

 discuss all the very numerous phenomena of movement 

 with which we have become acquainted, from the standpoint 

 of their biological significance ; to do that thoroughly, a 

 separate course of lectures would be required. But with the 

 help of the examples just given of the way in which various 

 kinds of irritability have been acquired, the elucidation of 

 other cases may be advantageously undertaken by the 

 student himself. 



BIBLIOGRAPHY. 



Contractile Vacuoles. 



Cohn ; Beitr. z. Biologic der Pflanzen, u, 1877, p. 118. 



Movements of Cellular Organs. 



Pfeffer ; Physiologische Untersuchungen, 1873. 



Lindsay; Quarterly Journal of Science, Literature, and Art, 1827, 



p. 76. 



Burnett and Mayo ; ibidem. 



Dutrochet ; Recherches sur la structure intime des Animaux et des 

 x, 1824, p. 59. 



