MORPHOLOGY 49 



full-grown plant take place in it, such as the preparation, conduction, 

 and storage of nutritive materials, water storage, and the process 

 of respiration. The presence of abundant cell sap contributes to the 

 maintenance of the general rigidity of the plant body. The structural 

 differences between parenchyma cells are relatively slight when the 

 multiplicity of functions they perform are considered. When the 

 cells have numerous chloroplasts they are spoken of as ASSIMILATORY 

 PARENCHYMA (Fig. 8) in reference to their main function of forming 

 organic substance from carbon - dioxide. The parenchyma of the 

 subaerial parts of plants is often of this nature so far in as light can 

 penetrate, while the deeper tissues are colourless. The term STORAGE 

 TISSUE (Figs. 23 A, 24) is applied when these cells are rich in 

 organic contents such as -sugar, starch, fatty oils or proteids, or have 

 hemicelluloses accumulated in the thickened walls (Fig. 39) ; these 

 substances are stored against future use in the metabolism. WATER- 

 STORAGE PARENCHYMA as a rule consists of large thin-walled cells with 

 little protoplasm but abundant cell sap that is somewhat mucilaginous ; 

 these cells diminish in size on losing water. Conduction of organic 

 food-materials, especially of carbohydrates, takes place in parenchy- 

 matous cells, which are elongated in the main direction of transport 

 to facilitate this function. Such CONDUCTING PARENCHYMA often 

 forms a sheath, without intercellular spaces, around other masses of 

 tissue. Parenchyma which has large intercellular spaces, serving for 

 ventilation or the storage of gases, is termed AERENCHYMA. 



2. Boundary Tissues. In the case of the multicellular tissues 

 composing the bodies of land plants the whole body or particular 

 tissues may require protection against excessive loss of water, 

 mechanical injury, excessive heat ( 40 ), and frequently against the loss 

 of diffusible substances. This function 

 is carried out by cells which have cer- 

 tain peculiarities of structure and are 

 often arranged in sheathing layers. 

 In this way another group of tissues 

 can be distinguished, the main elements 

 of which are the epidermal cells and 

 the suberised or cork cells. The epi- 

 dermis together with some other types 

 of cell form the epidermal system. 



(a) Epidermal System. 1. EPI- 



rpi , . , . , FIG. 43. Surface view of the epidermis 



DERMIS. 1 hlS IS derived trom the from the upper side of a leaf of Mercuri- 



superficial layer of the primary meri- aitsperennis. (x 300. After H.SCHEN-CK.) 

 stem (the dermatogen, cf. p. 86) and 



is thus one of the primary permanent tissues. It encloses the 

 plant body as a protective investment while permitting exchange 

 of materials with the environment. The epidermis is typically a 

 single layer (Fig. 45 B) of tabular or more elongated living cells, 



