544 MOTOR SYSTEM 



motor-tissues, while the fibrous layer of an anther and the turgescent 

 parenchyma of a pulvinus exemplify the active type. Active motor- 

 tissues may further be subdivided into two groups. The action of the 

 inanimate or mechanical variety is entirely dependent upon physical 

 changes, which take place either in cell-walls (imbibition mechanisms), 

 or in cell-cavities (cohesion mechanisms). The other group comprises 

 the living or physiological motor-tissues, which functionate by virtue of 

 the vital activity of their constituent cells. 



On comparing vegetable and animal motor-tissues with one another, 

 a striking contrast at once becomes apparent. The motor-tissues of 

 plants exhibit great diversity of structure and mechanism. This 

 diversity is mainly due to the fact that a variety of physiological pro- 

 cesses and anatomical arrangements have been adapted and developed 

 for the purpose of carrying out movements ; moreover, this adaptation 

 has not taken place at the same level of phylogenetic development in 

 every case. In the animal kingdom, on the other hand, the structure 

 and mechanism of the motor- tissues are comparatively stereotyped. 

 There, movement is almost invariably performed with the aid of (smooth 

 or striated) muscle-fibres which are derived, both phylogenetically and 

 ontogenetically, from specialised protoplasmic fibrillae. This uniformity 

 of the motor-tissues among animals is evidently correlated with the 

 fact, that the specialisation in question appeared during the earliest 

 stages of evolution, and has subsequently undergone a gradual and 

 continuous development. 



In the vegetable kingdom, special contractile protoplasmic organs 

 corresponding to muscle-fibres are, so far as is known, entirely absent, 

 principally, no doubt, because the prevalence of relatively rigid cell- 

 membranes does not admit of the employment of delicate contractile 

 protoplasmic fibrils for the purpose of performing movements. Plants 

 have thus, as it were, been compelled from the very first to make use 

 of cell-walls in the construction of their motor-organs. 



77. PASSIVE MOTOR-TISSUES. 



A. FLYING-HAIRS AND FLYING-TISSUES. 



The flying arrangements of wind-distributed seeds and fruits con- 

 sist either of air-containing trichomes, or of the thin sheets of tissue 

 which are known as " wings." The principal function of these 

 structures consists in increasing the friction between the air and the 

 falling fruit or seed, so as to prolong the sojourn of the latter in the 

 air ; as a result, the seed obtains a better chance of being carried to a 

 distant point by the wind. Where, as often happens, their flying- 

 organs expose a large surface to the wind, seeds may be carried over 



