8i6 
MECHANICS OF GROWTH. 
as soon as it emerges from the bud, and different stages of growth may be distin- 
guished in it, advancing from below upwards. This may take place in two different 
ways, according as the uppermost or lowermost part of an internode remains in an 
undeveloped condition, the other end being completely mature. This zone which 
continues for some time in an undeveloped state — cell-division taking place actively 
in it — is commonly found at the upper end of the internode (as in Phaseolus), and less 
frequently at the lower end, and this usually when it is enveloped by closely appressed 
leaf-sheaths or when it is in a bulb, as e. g. in Equisetacese (especially E. hyemale)^ 
Umbelliferae, the bulbous Liliacese, the haulms of Grasses, &c. If the internodes are 
not sharply defined, as in stems with small leaves and the floral axes of Dicotyledons, 
the various states of growth which have been described pass insensibly into one 
another on the stem ; and this is always the case with roots. If leaves when once 
expanded continue to grow for some time, the process is the same as in stems or 
branches ; while the lower portion of the leaf-stalk is fully mature, the upper parts 
present successively younger or less developed states. The formation of cells finally 
ceases at the apex and all the parts then become fully mature. This is strikingly 
the case in Ferns, less so in the pinnate leaves of Papilionaceae or the incised leaves 
of Araliaceae. But very often the activity of the punctum vegetationis of the leaves 
lasts for only a short time and its tissue completes its growth while cell-divisions 
still continue at the base of the leaf, and all the transitional states of growth are to 
be found between the base and the apex. This occurs, for instance, in the long 
leaves which grow from the bulbs of Liliacese and alUed Monocotyledons. When 
a cell-producing zone of this kind occurs at the base of an internode or of a leaf, 
with more mature tissue lying above it, the whole organ behaves as if this zone were 
a punctum vegetationis ; the states of growth succeeding one another in the reverse 
order. Such a zone, intercalated between mature portions of tissue, may be called 
an Intercatary Vegetative Zone. The growth of the internode or leaf may be termed 
basipetal, in contrast to the acropetal development where the punctum vegetationis 
lies at the apex of the internode or leaf. 
According as the conditions of growth — temperature, the supply of water, and 
illumination — are favourable, these phenomena proceed more or less rapidly and 
uniformly. Every young cell formed at the punctum vegetationis grows and matures 
more rapidly the more favourable these conditions are. But if the organs are 
observed under the most constant possible conditions as they emerge from the 
bud, it is seen that their growth, both in length and thickness, dependent on the 
gradual development of the cells, does not advance by any means uniformly. The 
growing portion of a root, internode, or leaf does not lengthen to an equal amount 
in equal consecutive intervals of time ; and the same is the case with stems con- 
sisting of a number of internodes, and with each zone, however small, of a growing 
organ. It is seen in fact that the growth of each part begins at first slowly, 
becomes gradually more rapid, and finally attains a maximum of rapidity, after 
which the growth becomes again slower, and finally ceases when the organ is 
fully mature. 
If successive equal intervals of time are represented by T^, Tg... T^, and the 
increments during these intervals by I^, I2, ... I,,, then it may be stated as a general 
rule that — 
