116 ANATOMY. 



between the surfaces of the cells ; but if the tissue is loose, the cells retiain their 

 rounded form, and leave larger or smaller intercellular canals (fig. 655). These 

 spaces occur between polyhedral cells when an interposed liquid or gas displaces 

 them ; and it may happen (fig. 655) that if a regular pressure is exerted in neigh- 

 bouring spaces, each of which is circumscribed by a small number of cells, the latter 

 may be disjointed, and a portion of their walls pressed inwards ; but where there are 

 two contiguous spaces, the pressures from without will counteract each other, and 

 the cells remain coherent ; they then take the shape of stars, the contiguous rays 

 forming isthmuses which separate the spaces. 



Sometimes the intercellular space is circumscribed by a great many cells ; it is 

 then called a lacuna. These lacunar do not always result from the displacement of the 

 surrounding cells, but from the destruction of several of them, or the rapid growth 

 of the plant. 



In their earliest condition cells are sacs surrounded by a thin homogeneous 

 membrane, which is soft and moist at first, but dries by degrees. Sometimes this 

 membrane constitutes the sole wall of the cell, sometimes it is lined by a second ; but 

 the latter does not form a continuous sac ; it is wanting here and there, and only 

 partially lines the outer membrane ; the result is that there are thin areas where 



656. Elder. 667. Mistleto. 658. Mistleto. 



Bayed CfeUs. Rayed and reticulate cell. Annular cell. 



there is but one membrane, and thick areas where there are two. When the inner 

 membrane is deficient only in small spots, these appear as punctures (fig. 652) or 

 short lines (fig. 656) ; when it is absent over considerable irregular areas, the thin 

 places form an irregular network (fig. 657), of which the open parts answer to those 

 where the inner membrane is wanting, and the threads to the parts where it lines 

 the outer membrane. Lastly, when the solution of continuity of the inner membrane 

 is extremely regular, the open spaces are separated by parallel thickened rings 

 (fig. 658), or a thickened spiral which passes from one end of the cell to the other 

 (fig. 659). Cells may either be homogeneous, or punctate, or rayed, or reticulate, or 

 spiral, or annular ; and in many cases the same cell passes successively through 

 more than one of these forms. It frequently happens that a third, fourth, or fifth 

 membrane is developed within the second, by which the wall of the cell is corre- 

 spondingly thickened. It has been observed that these successive membranes usually 

 mould themselves upon the second, so that the thin and thick portions of the cell 

 correspond throughout. 



Fibres. — The length of these varies, but most have a very thick wall, formed at 

 first of a single membrane, lined by a succession of others developed within it ; and 

 as the cavity of the fibre diminishes more and more with age, the fibre finally appears 



