The Substance Which Is AHve in Plants 155 



by the need for the presence of very large air spaces, for which a 

 branching, or stellate form is most suitable. The polyhedral 

 shape due to mutual pressures, in conjunction with the formation 

 of new walls as plates thrown across cells from one wall to the 

 other, results in the formation of cubical cells in growing points 

 (figures 49, 53, 139 CD), or elongations thereof to four-sided 

 prisms, as in the cambium cells, which form the growth zone 

 between the bark and the wood in most trees (figure 139 B). In 

 other cases the cells become flattened to tabular shapes, as in 

 epidermis and cork (figures 2, 49) ; where the function of those 

 cells as the protective skin of the plant obviously requires such a 

 shape. Again, the spherical or polyhedral shape becomes elon- 

 gated to a cylindrical or prismatic form where the function re- 

 quires much length, as it does in the conduction of liquids through 

 the plant; and it is a line of such cylindrical cells, thrown into 

 a tube by absorption of the intermediate walls, which constitutes 

 the water-carrying ducts, (figures 49, 53, 54 C, 72) while the food- 

 carrying sieve-tubes are made in analogous manner (figure 72), 

 Or, the elongation takes place at two opposite points, result- 

 ing in a spindle or fiber form, which is developed wherever tensile 

 strength for resistance to strains is required (figures 49, 50 d). Fi- 

 nally, through the intermediation of a more active growth at several 

 points, the spherical or polyhedral shape becomes modified to a 

 branching, or even a star-shaped form; and this occurs in the 

 spongy cells of green leaves as a means of providing generous 

 inter-cellular air spaces, (figure 2, and B of Plate I): in some 

 Rushes as a part of their very flexible pith (figure 49) ; and in 

 certain excretion cells of Water-plants as a means of providing 

 more wall for the deposition of waste crystals. Thus these few 

 ground forms, — the fundamental sphere, with its lines of modifi- 

 cation, shown by figure 49, — viz., ellipsoid-ovoid, polyhedral, 

 tabular, cylindrical-tubular, spindle-fibroid, and branched-stellate, 

 represent the mathematical possibilities upon which the cells can 

 play, but by which they are also bound in their adaptations to 



