CHAPTER VIII. 



INTEKCELLULAE SPACES AND SECEETIOlSr EES- 

 EEVOIES. 



160. — In addition to the caTities and passages which are 

 formed in the plant from cells and their modifications, there 

 are many important ones which are intercellular, and which 

 at no time were composed of cells. In some cases they so 

 closely resemble the cavities derived from cells that it is with 

 the greatest difficulty that their real nature can be made out. 

 In their simplest form they are the small irregular spaces 

 which appear during the rapid growth of parenchyma-cells 

 (Fig. 51, p. 67) ; from these to the large regular canals 

 which are common in many water plants there are all inter- 

 mediate gradations. 



161. — In leaves, especially in the parenchyma of the under 

 portion, there are usually many large irregular spaces be- 

 tween the cells ; they are in communication with the exter- 

 nal air through the stomata, and contain only air and watery 

 vapor. The petioles and stems of many aquatic plants con- 

 tain exceedingly large air-conducting intercellular canals, 

 which occupy even more space than the surrounding tissues 

 (Fig. 9, page 30). In the Water-lilies {Nyviplimacem) and 

 Water-plantains {AUsmacew) they are so large as to be read- 

 ily seen by the naked eye, and in the iNaiads {Naiadacem) 

 they are almost equally large (Fig. 114). In the flbro-vascu- 

 lar bundles of Equisetum, and of many Monocotyledons and 

 some Dicotyledons, there are intercellular canals, sometimes 

 of very considerable diameter (Figs. 99, 103, 103). Lastly, 

 in the medullary parenchyma (pith) of many plants there is 

 a large central cavity (although formed in part by the rup- 

 ture of some cell- walls), which must be considered as inter- 



