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F. C. STEWARD AND Η. Y. MOHAN RAM
and develop from the cambium of the secondarily thickened carrot root,
they form a very regular tissue. This is not so as the proliferated cells
grow around a carrot explant; the full range of form and growth of
which the individual carrot cell is potentially capable is only to be seen
in a medium competent to support its growth and in an environment
in which the cells can grow unconfined and unconnected. These events
have recently been described by Steward, Mapes and Smith (1958), and
the very suggestive features which emerge are as follows.
When granted a full supply of nutrients, the individual carrot cells
show a much greater range of growth and form than they exhibit when
present as units in a tissue. The cells now seem to respond to intrinsic
rather than extrinsic factors that determine their growth. Relatively
small isodiametric cells divide equationally, leading eventually to a
colony or clone of tissues while comparatively large, or giant, cells
after first becoming markedly multi-nucleate, eventually form a tight
compact mass of small potentially growing cells within the old cell wall.
From the single cell state the multi-cellular condition may be achieved
also by a filamentous type of growth or again by budding almost
analogous to that which occurs in yeast (Steward, Mapes and Smith,
1958). Even relatively small, densely cytoplasmic cells may divide in
ways that are now seen superficially to resemble divisions in sporogenesis.
However, the point is that in all these different ways individual cells
express their capacity to grow, and each route may lead eventually to a
multi-cellular colony of cells. The way in which these colonies organize
to form roots and then shoots, giving expression to the totipotency of
these cells is discussed in a subsequent section of this chapter.
Two principles, to which albeit there are many exceptions, have
guided thought on how plant cells divide; these are Sachs' Laws (1887)
that daughter cells tend to have equal masses, that new walls intersect
the old ones at 90°; and Errera's Law that the new wall or septum has
minimal area. Other general ideas have held sway, namely that cell
division is most prevalent in densely cytoplasmic cells with small
vacuoles, and that cell shapes often tend to resemble those that fill or
partition space efficiently. As D'Arcy Thompson shows (1952), Sachs'
generalization that new walls tend to intersect the old at right angles
applies only to situations in which the older walls have become more
rigid before the new ones form. So long as the existing walls are 'fluid'
or plastic and can flow, the angles of intersection where three walls meet
will tend toward 120°, but when they have become more rigid and solid
the new walls cross the old ones at right angles. Furthermore, the new
wall is usually highly curved when the two daughter cells which are cut
off are very dissimilar in size. Filamentous growths of carrot show these
traits in that divisions that occur along the filament are usually at right
F. C. STEWARD AND Η. Y. MOHAN RAM
and develop from the cambium of the secondarily thickened carrot root,
they form a very regular tissue. This is not so as the proliferated cells
grow around a carrot explant; the full range of form and growth of
which the individual carrot cell is potentially capable is only to be seen
in a medium competent to support its growth and in an environment
in which the cells can grow unconfined and unconnected. These events
have recently been described by Steward, Mapes and Smith (1958), and
the very suggestive features which emerge are as follows.
When granted a full supply of nutrients, the individual carrot cells
show a much greater range of growth and form than they exhibit when
present as units in a tissue. The cells now seem to respond to intrinsic
rather than extrinsic factors that determine their growth. Relatively
small isodiametric cells divide equationally, leading eventually to a
colony or clone of tissues while comparatively large, or giant, cells
after first becoming markedly multi-nucleate, eventually form a tight
compact mass of small potentially growing cells within the old cell wall.
From the single cell state the multi-cellular condition may be achieved
also by a filamentous type of growth or again by budding almost
analogous to that which occurs in yeast (Steward, Mapes and Smith,
1958). Even relatively small, densely cytoplasmic cells may divide in
ways that are now seen superficially to resemble divisions in sporogenesis.
However, the point is that in all these different ways individual cells
express their capacity to grow, and each route may lead eventually to a
multi-cellular colony of cells. The way in which these colonies organize
to form roots and then shoots, giving expression to the totipotency of
these cells is discussed in a subsequent section of this chapter.
Two principles, to which albeit there are many exceptions, have
guided thought on how plant cells divide; these are Sachs' Laws (1887)
that daughter cells tend to have equal masses, that new walls intersect
the old ones at 90°; and Errera's Law that the new wall or septum has
minimal area. Other general ideas have held sway, namely that cell
division is most prevalent in densely cytoplasmic cells with small
vacuoles, and that cell shapes often tend to resemble those that fill or
partition space efficiently. As D'Arcy Thompson shows (1952), Sachs'
generalization that new walls tend to intersect the old at right angles
applies only to situations in which the older walls have become more
rigid before the new ones form. So long as the existing walls are 'fluid'
or plastic and can flow, the angles of intersection where three walls meet
will tend toward 120°, but when they have become more rigid and solid
the new walls cross the old ones at right angles. Furthermore, the new
wall is usually highly curved when the two daughter cells which are cut
off are very dissimilar in size. Filamentous growths of carrot show these
traits in that divisions that occur along the filament are usually at right