Cells and Tissues 65 



Each cell of this mass is without a wall ; its protoplasm can 

 reach out in any direction and creep or stream from one 

 place to another. By placing some in a moist chamber this 

 streaming may be watched. Cells of higher plants have walls, 

 except some which are connected with reproduction. 



Cell walls are commonly of cellulose. This is a carbo- 

 hydrate which varies in different plants. Its chemical formula 

 is (CeHioOg),^. It is derived from the cell protoplasm with 

 which it is always in contact in a living cell, and when active it 

 is saturated with water. 



As cells mature they undergo a variety of changes. These 

 are alterations in chemical composition, in shape, and in 

 thickness of the wall. 



Chemically the walls may become lignified or woody, when 

 they become highly elastic and permeable to water, suberized 

 or corky, as in the outer bark ; such cells are impermeable to 

 water and air and are therefore fitted for protecting. Cutin, 

 occurring in the walls of the epidermis as in cabbage leaves 

 and those of Othonna and Protea, is of the same service as 

 suber. Walls may become mucilaginous. They swell and 

 become gelatinous when soaked in water, they serve to fix the 

 seed to soil in the case of linseed and cress, and while food 

 material is usually stored within the cell cavity, mucilage, e.g. 

 in tubers of Orchids, and the thick cellulose walls in the seed 

 of Coffee and Date, are used as reserve food. In Acacia a 

 further change occurs and the walls change to gum which dis- 

 solves in water. 



The shape of the cell which constitutes a whole plant in 

 itself may be round or nearly so, but when cells combine to 

 form tissues they become flattened, angular, and sometimes 

 much longer than broad. They may join each other by flat 

 surfaces or they may overlap by long pointed ends. Long 

 pointed cells are commonly found in wood and in the inner 

 bark. Such cells are called prosenchyma to distinguish them 

 from parenchyma where the cells are about equal in length 

 and breadth. 



Cells placed end to end may lose their end walls and form 

 long continuous tubes or vessels. Vessels always lose their 



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