CELL. 



[ 143 ] 



CELL. 



of such tissue in the particular group to 

 which the plant belongs, and not upon any 

 general law of globular expansion. The 

 above law does prevail widely in some fami- 

 lies, as in the Fungi ; and we very frequently 

 see it prevailing in pith up to a certain period : 

 but it will not hold as a general rule ; for the 

 lax tissues of leaves, of succulent stems, &c. 

 offer most striking deviations, as do also the 

 varied and often elegant forms of lower 

 Algae. It is frequently stated in books, 

 that the effect of pressure on cells having a 

 tendency to become globular, is the produc- 

 tion of a dodecahedral form ; but this again 

 is far too sweeping a generalization, and the 

 real fact is that globular cells of equal size, 

 expanding in a confined space, often become 

 twelve-sided by mutual pressure ; but far 

 more often the cells of a tissue are of diverse 

 size, and hence a polyhedral form is much 

 more common (tig. 111). Cells may be 



Fig. 111. 



globular, as in the Yeast-plant, and many 

 lower Algae, in the lax tissue of young pith 

 of many Dicotyledons (PL 47. tig. 14), &c. ; 

 oval, as is much more common in parenchy- 

 matous tissues; squarish, as in cork (PL 47. 

 figs. 16, 17) ; or tabular, as in the epidermis 

 of numerous plants, under w T hich circum- 

 stances the side walls may be square, 

 rhombic, hexagonal or irregular, as in many 

 petals ; and the outlines may 

 also be undulated or even Fig. 112. 

 beautifully zigzagged, as in 

 the leaf of Helleborus fcetidus 

 &c., the petals of many 

 flowers, or in the leaf of the 

 Pineapple (PL 47. fig. 15), 

 &c. ; while the upper exposed 

 face may be flat or vaulted, 

 as in most petals, or even 

 papilliform, as in the petals 

 of the Sweet William and 

 of most flowers with glisten- 

 ing surface. Cells may also be cylindrical, 

 and then either with flat ends (fig. 112), 

 as in the parenchyma of many Monocoty- 

 ledons and in the filaments of Confervas, or 



rounded ends or attenuated ends, as in wood 

 and liber tissue generally ; or they may 

 be prismatic, and then square or six-sided, 

 as in stems of most herbaceous plants ; 

 spindle-shaped, as in a large number of 

 woods, such as that of Conifers, Box, 

 &c. ; and, in fact, of almost every con- 

 ceivable form. In lax tissues, the walls 

 of the cells often grow very unequally at 

 different points, whence result angular pro- 

 jections, by which the cells ordinarily co- 

 here (fig. 113) ; or these grow out into 

 arms or rays, producing stel- 

 late cells, as in the pith of -p- -j -j o 

 the Rush (PL 47. fig. 18), 

 and the parenchyma of many 

 aquatic plants, in the leaf- 



stalk of the Banana, &c. 



Cells which are free, as in . y^ 



the lower cellular plants, "7 /\J^? j 

 sometimes grow out into ^\ ..-. j 

 long tubular structures such <J\J 



as Vaucheria, with a conti- 

 nuous cavity, and indeed sometimes ramify 

 into a complication of branches, as in ry- 

 opsis and Codium, while in Botrydium (fig. 

 75) the globular cell sends down a number 

 of root-like filaments which are mere pro- 

 trusions of its own wall. The cells of Chara 

 attain very large size. In the Flowering 

 plants we have an example of extraordinary 

 growth of a single cell in the* pollen-tubes, 

 which, in some cases, become as much as 

 three inches long. 



Size. The dimensions of cells vary to 

 infinity, and, indeed, often extremely in one 

 and the same tissue, but not as a rule. And 

 the diameter of cells is very frequently in- 

 comparably less than the length, as in all 

 filamentous and fibrous cells. Taking a very 

 general view, we may say that parenchyma- 

 cells vary from 1-250 to 1-1000" in dia- 

 meter ; but the spores of many Fungi mea- 

 sure no more than 1-6000 to 1-8000", while 

 the cells of the juicy parenchyma of many 

 fruits and piths attain as much as 1-100". 



The smallest cells appear to occur in 

 Palmella hyalina, where they measure the 

 1-83,000". In elongated cells, such as those 

 of liber and most woods, the diameter is 

 ordinarily less than in parenchyma, while 

 the length is far greater : thus in wood the 

 length varies from about 1-40 to 1-12'', 

 while the diameters are respectively 1-300 

 and 1-100"; in liber the length may extend 

 to 1-8 or 1-4", with a diameter of 1-800 and 

 1-400". (See FIBRES.) Hairs composed 

 of a single cell often attain a great length, 



