24 



BOTANY OF THE LIVING PLANT 



Fig. 14. 



Section of a cell of Hoya camosa, 

 with greatly thickened, stratified, 



as it stretches. As the limit of size of the mature cell is approached 

 and the stretching ceases, the thickening of the wall may be more 



rapid, and it is probably this which 

 causes the cessation of growth. The 

 thickening may be continued till in 

 extreme cases a large proportion of the 

 space within the original film of cell-wall 

 is filled up. Often this original wall may 

 be recognised in the mature state as a 

 " middle lamella," where two or more 

 thickened walls adjoin (see Fig. 16, B, 

 p. 27). In the mature cells with thick 

 walls the layers of stratification can 

 often be clearly seen (Fig. 14). But the 

 thickening of the walls is seldom uniform. 



and pitted walls. The pits are very p„ rfa : n Qrpc ,c arP Ipff- thin o nr | a note- 

 narrow, and often branched. (After Lertain areas are leit tnin, ana a note 



vonMohi.) worthy feature is that the thin areas in 



adjoining cells usually correspond. Such thin areas are called Pits, 

 and the partitioning wall is called the pit-membrane. Pits are of use 

 in facilitating the physiological communication between cells, and 

 practically all mature cells show pits of some sort on their walls 

 (Figs. 15 £, 16 B). 



(3) Changes of Substance of the Cell-walls. 

 In the young state the cell-walls are composed of a carbohydrate 

 substance, namely cellulose, together with more or less of those 

 pectin-substances which form the basis of fruit-jellies. Such cell-walls 

 are at first yielding and plastic like putty, but they become more 

 resistant and elastic as they grow older. As they become mature 

 the chemical and physical nature of the walls may change. Some 

 walls become lignified or woody, and are then mechanically more 

 resistant and harder. Such walls give its character to the wood of 

 tree-trunks. Others become suberised, or corky, and are then im- 

 pervious to the passage of water. Bottle-cork consists of masses of 

 dead cells with corky walls. Others may become gummy, or mucila- 

 ginous, and are liable to swell greatly on access of water, which they 

 thus retain. Gum arabic as sold in shops consists of hardened 

 amorphous masses of gum exuded from the stem of certain Acacias. 

 By such changes the cells may become fitted to perform different 

 specific functions in the mature parts. 



