CH. IV] MOTOR-CELLS 63 



Reference has been made to the rolling and folding of 

 leaves, due to the thin-walled cells on the upper surfaces 

 capable of varying in turgescence (motor-cells). These 



Fig. 21. Transverse section of left-half of leaf of Poa annua ( x about 50) 

 showing keel below, and two flanking lines of motor-cells (slightly 

 shaded) above the median vascular bundle of the mid-rib. Hence 

 the leaf folds. The half lamina has six smaller vascular bundles, 

 only the stronger one girdered. Eidges practically obsolete and 

 subtending bands of sclerenchyma slight : hence the leaf-surfaces 

 are parallel. 



are specially adapted epidermal cells found on the upper 

 surfaces only. In the leaves of Poa compressa, P. annua 

 (Fig. 21), P. nemoralis, P. alpina, Gatabrosa, Sesleria, &c., 

 a row of these motor-cells, easily distinguished by their 

 large size, thin walls and clear contents, is found on each 

 side of the mid- rib; as they dry the, leaf folds its two 

 halves together (conduplicate), and oA the re-absorption 

 of water they flatten the two halves out again. In 

 Bactylis these flanking rows coalesce into one over the 

 mid-rib. In other leaves, e.g. Avey\a pratensis, Festuca 

 elatior (Figs. 17, 22), Melica, Elymu^{Yig. 25), &c., there 

 are in addition to these two flanking rows, other sets of 

 motor-cells between the other ribs, and their combined 

 action causes the halves of the lamina to inroll, usually 

 one-half inside the other — convolute. 



It is easy to observe leaves of such grasses as Festuca 

 pratensis (Fig. 22), Aira ccespitosa (Fig. 23), &c., which 



