DIRECTIONS OF GROWTH, 
20^ 
base to the apex is the longitndinal direction of the member under consideration ; 
and a section made in this direction is called a longitudinal section. The transverse 
direction and section of the member are at right angles to the longitudinarones. 
(2) In every transverse section of a member there is a point about which the 
internal structure and external contour are so arranged that it must be considered 
as its Organic Centre. Every line drawn from this point towards any point of the 
circumference is a radius; every portion of the transverse section has one side 
facing the circumference and one facing the centre, these being usually developed 
in a different manner from the sides that face the radii, and hence easily distin- 
guishable from them. These relationships are recognised with ease in the transverse 
section of woody stems and of all roots, but can be easily made out in other 
cases also, even in unicellular plants and hairs. The organic centre of the trans- 
verse section does not usually coincide with the geometrical centre, as is easily 
seen in the transverse sections of most petioles and horizontal branches with an 
' eccentric' pith. 
(3) If a line be imagined uniting the organic centres of all the transverse 
sections of a member, this is the Longitudinal Axis or Axis of Growth of the 
member. The axis of growth may be a straight or a crooked line; in the younger 
parts (nearer the apex) it may be crooked, and again straight in those w^hich are 
further developed (further from the apex), as in Salvinia and Utricular ia ; or the 
reverse. A plane which passes through the member in such a manner as to contain 
the axis is called an Axial Longitudinal Section, If the axis be curved in a plane, 
this plane coincides with the axial longitudinal section ; if the axis is straight, the 
number of possible axial longitudinal sections is very large or even infinite. 
Growth in the direction of the longitudinal axis is generally quicker, and also 
generally lasts longer, than in the transverse directions, as is clearly seen in most 
stems (haulms, flower-stalks, scapes, palm-stems), in long leaves, in all roots, and 
in most hairs and thallomes. This characteristic cannot however be used in the 
general defmition ; for there are cases in which it appears doubtful whether the 
growth in the direction of the longitudinal axis is more intense or more prolonged 
than in the radial directions; as, e.g. in the stem of Lsoetes, and the prothallium of 
some Polypodiaceae. But the characteristic is superfluous for the determination of 
the longitudinal axis ; its direction can always be recognised by the position of the 
base and apex of a member ; and the point where it cuts the transverse section (the 
organic centre) can be found without anything else being known about the relation- 
ships of growth. It is always possible, without even knowing the duration or 
intensity of the growth, to decide which is the longitudinal and which the transverse 
section of a member ; this can indeed be determined from a very small fraction of 
it ; in a Mammilaria, a Melocactus, or a Cereus, it is just as easy to determine the 
longitudinal axis of growth in early youth, when these cacti are often as thick as 
they are long, as it is later when they are much longer than thick. This is also the 
case in the abbreviated axis of bulbs, in many tubers and corms (as the crocus), 
and in fruits, like those of many gourds, whose diameter is much greater than their 
length. 
The growth of roots and stems in the direction of their longitudinal axis is 
generally unlimited, that of leaves and hairs mostly hmited, although these rela- 
