50 LIVING PLANTS 



Organization 



ern greenhouses show an erect stem because 

 of insufficient light and are "drawn" in the 

 language of the gardener. The scattered 

 leaves consist of a long petiole bearing two 

 or four leaflets, which are divided into eight 

 to twelve pairs of small ovate pinnules. The 

 bases of the stalks of the pinnules, the leaflets 

 and the petiole are developed in the form of 

 thick cylindrical swellings (pulvini). The 

 woody, mechanical tissue in the stalks is in 

 the form of a hollow cylinder, but in passing 

 through thepulvinusto join the stem it comes 

 together forming a solid rod. The central 

 cylinder of mechanical tissue is surrounded by 

 a thick layer of thin walled motile cells which 

 are capable of rapid changes in form. Such 

 alterations are due to variations in the hydro- 

 static pressure in the cells. When the cells of 

 one side of the pulvinus give off water which 

 passes into the spaces between the cells, a 

 curvature toward this side results from the 

 unchanged pressure of the turgid cells of the 

 opposite side. The sinking of a leaf upon its 

 petiole is due to the relaxation of the cells of 

 the lower side of the pulvinus. When these cells 

 reabsorb the water from the intercellular 

 spaces, their former size and shape is slowly 

 regained and the leaf is returned to its former 

 position. 



