548 MOTOR SYSTEM 



The large satiny wings of Zanonia macrocarpa are also exclusively 

 composed of flying-tissue of a very remarkable type. The extremely 

 delicate margin of the wing consists of very thin-walled elongated 

 cells. A transverse section through the thicker central portion shows 

 that the two epidermal layers which constitute the actual flying-membrane 

 are linked together by loosely arranged, single-layered plates of tissue, 

 which are also connected among themselves by cross-ties (Fig. 223). 

 In a longitudinal section these buttressing plates are seen to consist 

 of tabular cells, which are mostly very thick-walled ; the terminal 

 elements in each buttress expand so as to provide a large surface of 

 attachment to the flying-membrane. The epidermal cells are likewise 

 thick-walled and cohere very firmly, owing to the fact that their 

 lateral walls are sinuous, and thus interlock ; the cells of the buttresses 

 are also frequently dove-tailed in a similar fashion. 



The wings of fruits often have a more complicated structure than 

 those of seeds, owing to the presence of vascular bundles {Ulmus), or 

 stout mechanical strands {Acer), in addition to the actual flying-tissue. 

 As Wahl has shown, such accessory tissues are always disposed in 

 accordance with the mechanical requirements of the wing. 269 



Where the wing is the morphological equivalent of a leaf, it usually 

 acts, to some extent, as a photosynthetic organ, at any rate while it is 

 young ; in such cases, the structure of the organ naturally embodies a 

 compromise between the demands of two different functions. 



B. FLOATING-TISSUES. 270 



The seeds and fruits of water- and strand-plants are generally dis- 

 persed through the agency of water-currents. It is, therefore, most 

 essential that the organs in question should be able to float for pro- 

 longed periods ; this necessity has led to the development and special- 

 isation of air-containing floating-tissues. Schimper discriminates between 

 several types of floating-tissue, which differ in structure, and in their 

 arrangement in the pericarp or testa. In some cases (Cerbera Odollam, 

 LagiMicularia racemosa, Nipa fruticans, Aegiceras majus) the tissue in 

 question contains many large intercellular air-spaces, and thus resembles 

 spongy parenchyma. More frequently, however, intercellular spaces 

 are feebly developed or altogether absent, and the air is contained in 

 cell-cavities. The cell-walls are thin, and often crowded with pits ; 

 they never consist of unmodified cellulose, but are often lignified, and 

 sometimes impregnated with substances of uncertain composition. A 

 very important property of such floating-tissues consists in the fact 

 that they are pervious to air, but very impervious to water, so that 

 they remain full of air for a long time, even in contact with water. 

 Schimper found that fragments of the floating-tissue of Barringtonia 



