BOTANY 



PAKT I 



which the closing membrane, in that case called the sieve-plate, is 



perforated by fine openings or pores (Fig. 85). 



In cases where the greater part of the cell wall remains unthick- 



ened, it is characterised rather by 

 a description of its thickened than 

 unthickened portions ; it is in this 

 sense that the terms annular, spiral, 

 and reticulate are used (Fig. 86). 

 Just as in the case of cells with 

 bordered pits, annular, spiral, and 

 reticulate cell walls are only 

 acquired by cells that soon lose 

 their contents, and act in the 

 capacity of water-carriers. Such 

 wall thickenings serve as mechani- 

 cal supports, to give rigidity to 

 the cells, and to enable the cell 

 walls to withstand the pressure of 

 the surrounding cells. Collen- 

 CHYMATOUS cells are living cells, 

 the walls of which are thickened 

 principally at the corners (Fig. 87). 

 Cells on the surface of plants 

 have usually only their outer walls 

 thickened (Fig. 100). By the 

 thickening of cell walls at special 

 points, protuberances projecting 

 in this way the formations known 



Fig. 88. — Part of transverse section of a leaf of 

 Ficus elastica. c, Cystolith ; c, e, <', triple- 

 layered epidermis ; p, palisade parenchyma ; 

 s, spongy parenchyma, (x 240.) 



into the cell cavity are 

 as CYSTOLITHS arise. 



formed : 



Certain large cells in the leaves of the Indiarubber plant (Ficus elastica) con- 

 tain peculiar clustered bodies, formed by the thickening of the cell wall at a single 

 point (Fig. 88). In their formation a stem-like body or stalk first protrudes from 

 the cell wall ; by the addition of freshly-deposited layers this becomes club-shaped, 

 and, by continued irregular deposits, itfinally attains its clustered form. 



So far only centripetal wall thickenings have been described. 

 Cells, the walls of which are centrifugally thickened, can naturally only 

 occur where the cell walls have free surfaces. The outer walls of 

 hairs generally show small inequalities and projections. The surface 

 walls of spores and pollen grains (Fig. 89) show a great variety of such 

 centrifugally developed protuberances, in the form of points, ridges, 

 reticulations, and bands of an often complicated internal structure. 



The Origin and Growth of the Cell Wall. — The cell wall is a 

 product of the protoplasm. When a previously naked protoplast, as a 

 swarm-spore of an Alga, envelops itself with a cell wall, this is 

 effected, as is now generally believed, by the transformation of its 



