180 



OUTLINES OF PLANT LIFE. 



growth (T 242) may also affect motor organs, producing like 

 curvatures. But elongation of any part of a motor organ by 

 increased turgor is reversible, 

 not permanent (cf. T 213) ; it 

 is therefore not growth. 



253. Motor organs. — The 

 motor organ in leaves is usually 

 the leaf base (T[ 124) or a modi- 

 fied portion of the stalk, some- 

 times greater but generally less 

 in diameter than the rest. Its 



ITlG. 133. 



Fin. 134. 



Fig 133.— Transverse sections through petiole of scarlet runner, .-1, through the rigid 

 portion ; B, through the motor organ, G, g, vascular strands ; c, cortex ; m, pith ; 

 r, deep channel along ventral side of petiole. Magnified about lo diam. — After Sachs. 



Fig. 134. —Portion of a scarlet runner, which, originally growing erect, has been inverted 

 for several hours, resulting in geotropic curvatures of the primary motor organs /*, P^ , 

 h'^. The lowest pair of leaves show secondary motor organs at the juncture of petiole 

 and blade. Similar ones are present in the upper compound leaves, but are not clearly 

 shown in the figure. The arrows show the position of the petioles when the plant was 

 first inverted. About two thirds natural size.— After Sachs. 



cortex consists of large cells, and the stele occupies a rela- 

 tively small part of the transverse section. In other parts of 

 the petiole the stele is much larger, or there may be several 



