68 



BOTANY 



I'AUT I 



are only acquired by cells that soon lose their contents, and act as 

 water-carriers. Such wall thickenings serve as mechanical supports, 



to give rigidity to the cells, and to 

 enable the cell walls to withstand 

 the pressure of the surrounding 

 living cells. 



The thickened bands by which 

 thickenings of the cell wall are effected 

 an: attached by narrowed bases to the 

 primary membrane ( 49 ). When the mem- 

 Inane separates two water - conducting 

 elements its central portion is thickened 

 like the torus of the bordered pit. The 

 annular and spiral types of thicki-niiii: 

 characterise the water -conducting ele- 

 ments of growing parts of the plant, 

 >ince they allow of extension of the wall 

 iluring growth. The thickening band 

 can often be removed as a continuous 

 spiral from the lamella to which it i* 

 attached. 



COLLENCHYMATOUS cells are 

 Fu, 7G.-l>art of transverse section of a leaf of lg cells, the Walls of which are 



Fieus dastica. c, Cystoiitu ; e, , e, three- thickened principally at the corners 



layered epidermis: ;. ? palisade ^renchyma : /pj g 74 c \ suc ^ ce ]J g occur com . 

 *, spongy parenchyma, (x '-'40.) , . /, , . , , . , 



monly in the more highly organised 



plants and form a special kind of tissue (p. 116). Cells on the 

 surface of plants have usually , only their outer walls thickened 

 (Fig. 74 e). 



Unequal thickening of the cell- wall 

 is most noticeable when it is limited 

 to definite small areas. This is the case 

 with those lens-shaped thickenings of 

 the outer walls of the epidermal cells 

 of the upper surface of foliage leaves, 

 which are of frequent occurrence and, 

 according to G. HABERLANPT ( M ), 

 act as convex lenses to concentrate 

 the light (Fig. 75). By the thicken- 

 ing of cell walls at special points, 

 protuberances projecting into the cell 

 cavity are formed ; in this way the 

 formations known as CYSTOLITHS 

 arise. Certain large cells in the leaves 

 of the India-rubber plant (Ficus elas- 

 tica) contain peculiar clustered bodies, formed by the thickening of the cell wall 

 at a single pointj (Fig. 76). In their formation a stem-like body or stalk first 

 protrudes from the cell wall ; by the addition of freshly deposited layers this 



Kn;. 77. -.1, I'nlleii-s.'i-ain nt Cucurbita I 1 '/-" iii .sin-- 

 face view, and partly in uptiral sertiiin. rendered 

 transparent I'.v treating with oil of lemons 

 (x 240); R, part of transver-r src-tinn of pollen 

 .urain of Cifurliittt re rnif'Sa ( x ">4<i). 



