INTRODUCTION TO PLANT HISTOLOGY 



455 



also, notably in the pith of the Rush, Jiincus, there are six-rayed, stellate, 

 parenchyma cells attached only at the ends of the arms (Fig. 442). This 

 appearance is due to local wall growth 

 in the pith cells, which are held within 

 a ring of resistant external tissues 

 which have ceased to grow. As out- 

 ward expansion is prevented the free 

 portions of the cell walls fall inwards 

 into catenoid curves. 



Occasionally we meet parenchyma 

 cells with thickened walls. The most 

 important case is that of coUenchyma, 

 a tissue often found in the outer layers 

 of stems and petioles, especially where 

 ridges or angles exist. The cell walls 

 are heavily thickened with cellulose, 

 which may be regularly developed, 

 but is usually irregular, forming 

 especially in triangular masses, or 

 trigones, where three cell walls meet. 

 This seems to be a means of increas- 

 ing mechanical strength. 



Parenchyma cells may also become lignified, that is, the walls receive a 

 deposit of lignin or hard woody matter (see p. 460). This often happens in 

 old pith and in the parenchyma of the woody vascular tracts, or it may affect 

 only small groups of cells embedded in soft tissues. The thickening of the 

 wall is sometimes extreme, very little lumen being left unfilled. The lignin 

 is laid down in concentric layers and is traversed by narrow, radial canals, 

 often branched, which connect the wall to the central lumen, and are in 

 fact pits, or more correctly pit-canals, leading to pits in the middle lamella. 

 Cells of this kind are called stereids, or stone cells. They occur in many 

 soft tissues, especially in fruits. A well-known example is in the flesh of the 

 Pear. Stereids of extraordinarily irregular form are not uncommon, as, for 

 example, in the cortex of many Magnoliaceae. 



Among specialized forms of parenchyma cells we should also include the 

 cells of the epidermis and endodermis and of the cork. 



Fig. 442. — Juncus effusus. Transverse 

 section of stem showing stellate 

 parenchyma in pith. 



The Epidermis 



The epidermis forms a continuous outer sheath over the exposed parts 

 of plants, but in older parts it is usually replaced by the secondary bark. 

 It is rarely more than one cell thick, except in tropical plants exposed to 

 powerful sunlight. The cells are usually small and often of a rather flattened, 

 tabular form (Fig. 443). They are living cells, but they very rarely contain 

 chloroplasts. The inner walls may be thin or thick, but the lateral walls are 

 thin and are usually provided with numerous large pits. Water can thus 



