552 



MOTOR SYSTEM 



contraction of the two lamellae almost always cross one another at 

 right angles ; in this way the requisite antagonism between the two 

 sides of the peristome-tooth is ensured. 



Certain seeds and fruits are furnished with hygroscopic flying- 

 hairs, which curl inwards from the base when wetted, so as to become 

 closely appressed to the surface of the fruit (or to the style or 

 pappus-ray, as the case may be), whereas in dry air they spread 



outwards. At the base of each hair 

 there is a more or less extensive 

 hygroscopic zone, in which the cell-wall 

 consists of transversely seriated micellae 

 on the outer or convex, and of longi- 

 tudinally seriated particles on the inner 

 or concave side. Not infrequently, the 

 fact that the side of the hair which 

 contracts most strongly on drying is 

 made up of transverse micellar rows, 

 can be inferred, without any examina- 

 tion of the optical properties of the 

 membrane, from the presence of trans- 

 versely elongated slit-shaped pits. In 

 Dryas Drummondi, Anemone Pulsatilla, 

 etc., a cushion-like enlargement of the 

 more highly contractile side of the wall 

 increases the intensity of curvature 

 Fio.224. (Fig. 224). 



In other instances, the antagonism 

 between the two sides of the hygroscopic 

 organ is clue to their being composed of 

 different tissues. The involucral bracts of the ripe capitula of Centaurea 

 and certain other Compositae curl inwards in damp, and outwards in dry 

 air, thereby preventing the wind-dispersed fruits from escaping during 

 rainy weather. The following description of the thick-walled motor- 

 tissue of the bracts of Centaurea is based on Steinbrinck's account. 

 Immediately beneath the outer surface which, on drying, becomes the 

 concave side is situated a layer of prosenchymatous cells with 

 transversely elongated pits ; from what has been said above, it is 

 evident that this layer will swell more actively in the longitudinal 

 than in the transverse direction ; conversely, on drying, the most 

 pronounced contraction will be longitudinal. The second layer^ 

 reckoning from the outside is parenchymatous in character ; its walls 

 are also transversely pitted, but have a smaller power of imbibition 

 than those of the first layer. The following layer is likewise 



Flying-hairs. A. Dryas Drummondi. 

 B. Anemone Pulsatilla. (Only the basal 

 portions of the hairs are shown.) 



