THE AERIAL AND SECRETORY SYSTEMS. 523 



plasm, &c., for the nutrition of the plant, and their walls have a cellulose 

 reaction, become blue, not yellow, by addition of iodine and sulphuric 

 acid, &c. 



The ordinary position of the liber has been above stated, but it may also 

 occur in the interior of the bundles, in the medullary sheath (see Anatomy 

 of Stems), even in the midst of the wood. The liber, once formed, may 

 cease to grow, or it may retain more or fewer cells still endowed with 

 the property of dividiDg. 



The Aerial System. In most parenchymatous tissues of the 

 higher plants we find the cells so disposed as to leave passages of 

 greater or less capacity between them, which passages are usually 

 found filled with air, apparently secreted from the contents of the 

 cells. In imperfect parenchyma (fig. 521) these intercellular 

 passages occupy a very considerable portion of the space filled 

 by the tissue, and they intercommunicate in all directions. The 

 spongiform cellular substance of leaves is traversed by large 

 passages of this kind (fig. 585), expanded in many places into 

 air-spaces, forming a continuous system of cavities, which are in 

 direct communication with the external air by the stomata. When 

 stellate cellular tissue exists (fig. 521), the air-spaces are very 

 extensively developed. 



No intercellular passages or spaces exist in young tissues ; they are 

 subsequently formed by the cells separating from, each other as they 

 expand, and excreting air into the interspaces. 



Air-canals are long tubular channels, in petioles (Nymphae- 

 aceaa) or steins (Hippuris, Potamogeton, &c.), bounded by a cellular 

 wall, and generally arranged in a definite manner in the organs in 

 which they occur. They are sometimes continuous through long 

 tracts of the stems or petioles (NymphseaceaB), or they are sub- 

 divided into chambers by cellular diaphragms occurring at intervals 

 (petioles of Musa, stem of Hippuris, Myriophyllum, &c.). 



Lacunas are formed by this cellular tissue being torn down and 

 destroyed by expansion of the surrounding tissue ; examples of 

 this occur in the fistular stems of TJmbelliferse, which when young 

 have a solid pith ; but this is torn away by the expansion of the 

 cylinder of fibro-vascular bundles, and leaves a tubular cavity. The 

 hollow stems of Grasses, of Equisetaceae, &c., originate in the same 

 way. 



Secretory System. The structures in which are found the 

 substances usually called the secretions of plants consist of latici- 

 ferous vessels (see ante, p. 513), glands, reservoirs and canals for 

 peculiar secretions (resins, oils, &c.), and the so-called milk-vessels. 

 They for the most part occur only in particular plants or particular 

 organs, and present many special modifications in different Natural 

 Orders, occurring on the surface or infche interior as single cells or 

 in groups, or as simple or branched tubes, or in layers, but, how- 



