GROWTH AND MOVEMENT 129 



Two factors may be recognised which co-operate in determining the 

 contour of the developed Plant. One is the mode of growth inherited 

 from the ancestry. This is no doubt the chief and decisive factor 

 determining plant-form. It provides those distinctive characters by 

 which organisms are recognised and classified. But an important 

 second factor is the impress of the environment during groipth. Changes 

 of form and proportion may be induced by external influences which, 

 though they are not heritable, may appear very profound. They are 

 styled fluctuating variations. They may in extreme cases so modify 

 the vegetative development of the individual plant, that there may 

 be some difficulty in allocating it to its true systematic position till 

 its propagative organs have been examined. This applies especially 

 in the case of amphibious species, such as the Water Buttercups 

 in which the submerged leaves diflfer greatly in form from those 

 developed in the air. 



Movements in Mature Organs. 



Movements also occur in mature parts, though not so generally as 

 in growing organs ; and perhaps they are less distinct from these in 

 their nature than they seem to be. They also are directly referable to 

 stimulation of the protoplast. But the reaction depends upon 

 changes in turgescence of the mature cells, accompanied by shrinkage, 

 followed by recovery. The Sensitive Plant [Mimosa pudica) is a notable 

 example. It is a shrubby tropical weed, with doubly pinnate leaves, 

 which show obvious changes of position of the leaf and segments 

 according to circumstances. They, in common with the leaves of 

 many other Leguminosae such as the Common Clover, change their 

 position before night-fall, and at daybreak, assuming what are called 

 the night and day positions. During the day the leaf-stalk is erect, 

 with all its leaflets widely expanded in a horizontal plane (Fig. 90, /). 

 But in the evening the pairs of leaflets fold their upper surfaces 

 together, the pinnae instead of radiating widely, narrow the angles 

 between them, while the petiole falls from the erect to a pendent 

 position. In fact the whole appearance of the shoot alters (//). These 

 changes come about by a hinge-like action at the base of each segment 

 or pinna, and of the leaf-stalk itself. The vascular tissue is there 

 contracted to a compact strand, surrounded by a broad cortical tissue 

 with cellulose walls and active protoplasts, which is the motor tissue. 

 The changes depend upon the turgescence of these cells. When a 

 difference of turgescence occurs so that it is greater on the lower side, 



B.B. I 



