776 MECHANICS OF GROWTH. 
of assimilated food-material, water, oxygen, and a sufficiently high temperature. 
Under these conditions individual cells or masses of tissue may grow, provided that 
their organisation permits it. But independently of these conditions there are others, 
as we have seen in the last chapter, which, without absolutely causing or arresting 
growth, nevertheless influence it; as light, gravitation, and pressure. The first- 
named may be called the necessary, the last, the secondary conditions of growth. 
In all growth all the necessary conditions must concur while the secondary conditions 
intervene only in certain cases, and exert their modifying influence very differently 
on the corresponding parts of different plants. 
The conditions spoken of as Necessary and Secondary depend upon the en- 
vironment of the plant, and act upon it from without. They may therefore be 
described as External Conditions or causes of growth, in contradistinction to the 
Internal Cojtditions dependent on the organisation of the plant. The existence of 
the latter conditions is most strikingly manifested in the fact that all parts of plants 
are able to grow only during a certain time ; when this time — the period of youth 
and development — is past, they no longer grow, even when all the favourable 
conditions concur. This shows that the internal organisation undergoes changes, 
which at length render the continuance of growth impossible. But even in organs 
which are still growing a certain independence of external circumstances may be 
perceived ; an Oak-leaf invariably grows differently from an Elm-leaf, an Oak-fruit 
from an Oak-root. The differences of these processes of growth is at once manifest 
in the difference of form and of the other properties of the organ; and no com- 
bination of external circumstances has the power of giving to a root, by change in 
its growth, the form of a leaf or to an Oak-leaf the structure of an Elm-leaf. There 
are also certain internal conditions of growth which do not decide, like the age of an 
organ and the necessary external conditions, whether growth shall take place, or at 
what rate; but determine how it shall proceed, and what specific and determinate 
organisation shall be attained by it. This latter circumstance depends only on the 
parent plants, or in other words on the species or variety to which it belongs. 
Descent determines the specific character of the growth ; all the other conditions 
determine only whether growth shall take place at all, and with what rapidity and 
energy. The innate internal conditions that regulate the nature of the growth of 
the plant, when once present cannot again be destroyed or reversed ; while the ex- 
ternal conditions may be at one time brought into action, at another time set aside. 
The internal and external conditions of growth may therefore be distinguished as 
the historical and the physical ; but those properties of a plant which have -been 
obtained historically are generally termed hereditary. This expression is not open 
to objection unless heredity be considered, as has recently been done by many, 
as a kind of natural force requiring no further analysis. For in distinguishing 
hereditary conditions of growth — i.e. those that have been acquired historically — 
from physical ones, it is not meant that the former do not also owe their existence 
to physical causes, but only that besides the accidental concurrence of physical 
conditions, it is also necessary to take into account certain characters which the 
plant has acquired when in the embryonic condition (in the broadest sense of 
the term) in the form of definite specialities of organisation through the influence 
of its parents. 
