262 A TEXT-BOOK OF BOTANY. 



called, may not only occur on the inner surface of the wall, when 

 it is spoken of as CENTRIPETAL, but may also take place on the 

 outer surface, when it is known as CENTRIFUGAL, as in the spores 

 of lycopodium and the pollen grains of the Compositae. 



FORMS OF CELLS. 



Upon examining sections of various portions of the plant, it 

 is observed that not only do the cell-contents and cell-wall vary 

 in composition, but that the cells are of different forms, depend- 

 ing more or less upon their functions. Groups of cells which 

 are similar in form and function constitute the various tissues of 

 the plant; and include: (i) parenchyma cells, (2) mechanical 

 cells, (3) conducting cells, and (4) protective cells. 



Parenchyma. Under the head of parenchyma are included 

 those cells which are nearly isodiametric and thin-walled, the walls 

 consisting of cellulose lamellae (Fig. 134). They may contain 

 both protoplasmic and non-protoplasmic cell-contents. Accord- 

 ing to the function and nature of contents, five kinds of paren- 

 chyma cells are recognized : (a) CHLOROPHYLL-PARENCHYMA or 

 assimilation parenchyma contains numerous chloroplastids and 

 occurs in leaves and all green parts of the plant, (b) RESERVE 

 PARENCHYMA occurs in seeds, roots, rhizomes, leaves, and contains 

 starch, aleurone grains, fixed oils, and other reserve materials. In 

 some instances the parenchyma, as in the endosperm of date, 

 ivory nut, etc., may be very thick- walled (Fig. 135). The paren- 

 chyma in stems and leaves of various of the orchids, as well as 

 that of plants of arid regions, which store water, may be included 

 in this group, (c) CONDUCTING PARENCHYMA is found either 

 associated with the sieve or with the tracheae, the cells of the 

 phloem conveying the plastic substances, while those of the xylem 

 convey water and salts. The cells of the pith and cortex are, 

 as a rule, not utilized for the rapid translocation of food materials 

 to far distant parts, although every living cell and every tissue 

 has a certain power of translocation, and no doubt different 

 parenchymatous tissues exhibit varying degrees of functional 

 activity and differentiation. Thus large quantities of reserve 

 materials are rapidly transferred to the developing embryo through 

 the cells of the endosperm, and in young seedlings further trans- 



