486 BOTANY 



the petiole of a rapidly growing leaf of Victoria regia elongated over 

 12 centimetres in the course of an hour ! 



Periodicity of Growth. The rate of growth is affected by the 

 external factors. In general, growth in length is accelerated by in- 

 creased temperature, and diminished by increased illumination, but, 

 aside from these, there does not appear to be a regular periodicity in 

 the growth. 



Mechanism of Growth-movements. Aside from the movements due 

 to simple elongation, the movements of growing organs, circumnuta- 

 tion, opening of flowers, etc., are caused by unequal growth. In a 

 growing apex, if the growth is greater alternately on opposite sides 

 of the stem, there will be an oscillation, right and left ; but if, as 

 more commonly is the case, the zone of growth follows the sun, or 

 retreats from it, the result is an elliptical or circular orbit. 



In the leaves of Ferns, and the coiled inflorescence of Myosotis, 

 the growth is much greater upon the outer side, but the organ 

 straightens out as the growth becomes greater upon the inner side. 

 The opening of a flower-bud is accomplished by accelerated growth 

 upon the upper side of the base of the petals, thus pushing them 

 outward. Whether the extremely rapid opening and closing of such 

 flowers as the Tulip and Crocus are referable entirely to growth, may 

 be questioned. It is highly probable that temporary changes in the 

 turgescence of the cells concerned are also an important factor in 

 the process. 



Where an organ is dorsiventral, like the petal of a flower, the 

 extra growth upon the upper side is called "Epinasty," upon the 

 lower side, "Hyponasty." 



Movement of Variation 



Besides the movements due to growth, all organisms show move- 

 ments which are independent of growth, and are induced by various 

 stimuli. The mechanism of these movements is usually to be found 

 in changes in turgor of cells in the motile parts. Unicellular organ- 

 isms may show active movements either by means of cilia or pseu- 

 dopodia, and these are affected by the same stimuli which are 

 efficient in the higher plants. 



Among the latter, the movements of so-called " sensitive " organs, 

 like the leaves of Mimosa pudica and Dionaea, the sensitive stamens 

 of Berberis and Sparmannia, the stigmatic lobes of Catalpa, Tecoma, 

 and Mimulus, are the most striking. The sleep-movements of many 

 leaves, especially of Oxalidacese and Leguminosse, come under the 

 same category. These motile members may be provided with a 

 special organ, the Pulvinus, which is especially well developed in the 

 motile leaves of many Leguminosae. This is an enlargement of the 



