SECT. I 



MORPHOLOCxY 71 



or amorphous form. Blood-coloured leaves, sucli as those of the 

 Copper Beech, owe their characteristic appearance to the united 

 presence of green chlorophyll and anthocyanin. The autumnal 

 colouring of leaves due to the reddening of the cell sap has already 

 been considered in connection with the colour of the chromatophores. 

 The different colours of flowers are due to the varying colour of the 

 cell sap, to the different distribution of the cells containing the 

 coloured cell sap, and also to the different combinations of dissolved 

 colouring matter Avith the yellow, orange, ar red chromoplasts and the 

 green chloroplasts. There is occasionally found in the cell sap a 

 yellow colouring matter known as xanthein ; it is nearly related to 

 xanthophyll, but soluble in water. 



External Products of the Protoplasts. — Under this head may 

 be considered the cell-walls which characteristically enclose the 

 protoplasts of vegetable cells. 



Origin and Structure of the Cell Wall C^^). — The membrane which 

 encloses the vegetable protoplast is a product of the protoplasm. 

 Many low organisms belonging to the Algae liberate naked protoplasts 

 from their cells ; these swarm-spores (Fig, 98 A) serve to multiply 

 the plant vegetatively. They soon settle down, form a thin cell 

 membrane on the surface of the protoplast, and proceed to give rise 

 to a filament. In more highly organised plants the ovum, from which 

 the development starts, has no cell Avail until it has been fertilised ; 

 from this stage on, all the cells composing the plant are surrounded 

 by cell walls. At the growing points of plants the cells are separated 

 from one another only by extremely thin membranes or cell walls. 

 As the cells increase in number by repeated division, new cell walls 

 are being continually introduced between the existing ones. The 

 rapid growth in length which sets in a short distance from the 

 growing point, as a result of the increase in the size of the cells, must 

 be accompanied by a corresponding growth in surface of the cell 

 walls. So long as this growth in surface continues, the cell walls 

 remain thin. After the cells have attained their ultimate size, the 

 GROWTH IN THICKNESS of the cell walls begins. The growth in 

 surface of the cell wall may either involve the introduction of new 

 material, or may take place without this. In the latter case the mem- 

 brane would become thinner if new lamellae were not simultaneously 

 applied to the surface. Growth of the wall by the introduction of 

 new particles between those previously existing is termed GROWTH BY 

 INTUSSUSCEPTION, while that which occurs by the laying down of new 

 lamellae on the surface of the older ones is called growth by apposi- 

 tion. The later growth in thickness of most cell walls takes place 

 by apposition, and thus the stratification, which such thickened walls 

 exhibit, is brought about (Fig. 72). Thicker, dense layers alternate 

 with thinner less dense ones. The denser layers can be recognised 

 by their high refractive power. In many cases lamellae, deposited 



