74 HISTOLOGY AND PHYSIOLOGY. 



the nucleus has not been detected in the cells of all the 

 lower plants, though never absent in the higher ; in very 

 young tissue the cell-sap is not differentiated. From the 

 protoplasm the other parts are secreted or formed ; and 

 while the protoplasm remains, the cell continues to live. 

 With age, the cell-wall often thickens, as in woody tissue, 

 and the protoplasm disappears, immediately after which 

 growth ceases. 



96. The cell-wall, which is very thin in young cells, 

 often increases considerably in thickness. This thickness 

 is seldom, if ever, uniform throughout the whole wall, and 

 there results, therefore, a variety in its sculpturing. The 

 direction of the thickening may be centrifugal, or out- 

 wards ; or it may be centripetal, or inwards. The first is 

 illustrated by the projections, ridges, etc., on isolated or 

 exposed cells, as in many spores, pollen grains, etc. (Fig. 

 168). The centripetal thickening is seen in ordinary 

 woody tissue, whose elongated cells are compacted together. 

 The portion thickened may be in the form of rings (Fig. 

 173, an), when the thickening is said to be annular (Lat. 

 annulus, ring). The thickened band may be spiral (Fig. 

 173, sp), and either closely or loosely wound. When the 

 tissue is rudely torn asunder, these spiral bands often 

 uncoil, the thin portion of the wall giving way. The 

 spiral thickening is the most common. The reticulate 

 wall has thickenings so as to present a net-work (Fig. 173, 

 ret). In the scalariform (Lat. scalaria, flight of stairs, 

 or ladder) the thickening is at the longitudinal angles of 

 the cells, and extends across, so as to give approximately 

 the appearance of rounds of a ladder (Fig. 173, sea). 



97. When the thickening of the cell-wall is intercepted 

 at numerous isolated spots, it becomes pitted (Fig. 173, 



