108 



The movements hitherto considered are those which are 

 brought about chiefly by the unequal growth of opposite sides 

 of the organ concerned and hence they are only possible in parts 

 which are still growing. Those organs which have completed 

 their growth therefore have become fixed in position, and this 

 position isthe one which is onthe whole the best suited for the 

 performance of the functions of each individual organ, having 

 regard to the conditions of its environment. Thus a leaf which 

 is no longer able to move according as the direction of the light 

 rays falling on it changes, takes up a fixed position in which 

 it receives the full benefit of the greatest quantity of the 

 most suitable light rays. 



Movements In addition, however, to the movements which are brought 

 a ^>out by a difference in the rate of growth of different parts of 

 organs, plants are also in some cases able to effect the movement 

 of organs after the latter have completed their growth, in res- 

 ponse to contact, light, temperature, and other stimuli. In these 

 cases the movement is brought about by an alteration in the 

 turgidity of the cells on opposite, sides of the organ concerned. 

 In foliage leaves the pulvinus and pulvinule are organs especially 

 adapted for effecting movements of this description, which can 

 be well seen in the leaves of Mimosa pudica. If the cells on 

 the upper side of the pulvinus are turgid, while those on the 

 lower side are flaccid, the leaf will move downwards, and vice 

 versa, the leaf in each case turning on the pulvinus like a hinge. 

 As cells are only capable of becoming turgid when the cell walls 

 are elastic and not rigid, we find that the greater part of the 

 pulvinus consists of parenchymatous cells with non-lignified 

 elastic cell-walls, the vascular strands and strengthening 

 tissue being united in a central strand where it offers least- 

 resistance to bending, instead of being distributed nearer the 

 circumference. 



Eeproduc- 97. There are two modes of repro- 



SS' AseS 1 Auction m tne Vegetable Kingdom known as the Asexual, or 

 Methods. Vegetative) and the Sexual, respectively. In the former a portion 

 of the protoplasm of the parent plant, which may be a single 

 cell, or a multicellular structure such as a bud, separates from 

 the parent and, either at once, or after further growth, consti- 

 tutes a new individual plant. In sexual reproduction a new 

 individual plant can only be produced after the union of two 

 pieces of protoplasm has taken place which have been developed 

 on different plants, or on different parts of the same plant. 

 Several plants exist which propagate themselves only by the 

 asexual method, others are found to do so only by the sexual 



