ON THE POWER OF MOVEMENT IN PLANTS. 151 



some sort. We also usually find either some peculiarity in tissue- 

 co?istrnction^ or in the tmion of the moving organ to some other 

 organ, such as the stem, etc. 



Heat and light usually play a very important part in. Periodic 

 movements ; for example, in the Mimosa^ or Sensitive plant, 

 movement is not seen at a temperature under 15° C, while death 

 occurs if we get above 52^0. Prolonged darkness causes rigidity, 

 and therefore loss of motile power. 



As the most notable kind of Periodic movement we may take 

 the so-called Sleep of plants, or of plant-organs. 



I. — Sleep of Leaves. Mimosa^ or Sensitive Plant. This leaf is 

 bi-pinnate, i.e.^ the compound leaf is divided into four pinnce. or 

 leaflets, arranged like the sprays of a feather on the main axis, and 

 again, each pinna is divided into a series of pmnules similarly 

 situated. At night the pinnules fold upwards on one another all 

 along the axis, like butterfly-wings: the pinnae — />., the four leaflets 

 move up and laterally, closing over one another like the flaps of a 

 fan ; then the whole leaf-stalk bends downwards, getting closer to 

 the stem. In fact, a steady but general collapse takes place. 



These movements are effected, Darwin shows us, in two differ- 

 ent ways : first, by alternately increased growth on the opposite sides 

 of the leaf, leaflet, or leaf-stalk, as the case may be, the alternation 

 usually being in the stalk at some part or other ; this increase of 

 growth on one side, or on the other, is preceded by an increase 

 in the turgescefice of their cells ; secondly (and in our specimen 

 probably, except in the very young plant, the only method), by 

 means of an aggregated mass of cells called a pulvi?ius. The cells of 

 this pulvinus have no chlorophyll, and form a little lump or 

 swelling at the articulatiojt of the leaf-stalk with the parent stem^ 

 consisting of a vascular bundle wrapped round with soft parenchy- 

 matous tissue ; they exist also at the articulations of the leaflets 

 with the common leaf-stalk, and there is a separate one for each 

 pinnule, or ultimate leaflet, where it joins its common axis ; the 

 swellings here are called strtwice. 



These swellings act as follows : the parenchymatous cells, 

 which are the irritable ones, fill with water drawn from the plant ; 

 their ctW-walls are not irritable, but they are elastic. Let the cells 

 be irritated by a touch, or by concussion, or by degree of light or 



