DIRECTIONS OF GROWTH. JJOJ 



base to the apex is the longitudinal 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 longitudinal ones. 



(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 which are 

 further developed (further from the apex), as in Salvinia and Utricularia ; 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 definition; 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 oi Isoetes, and the prothallium of 

 some Polypodiacese, 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 

 ; in a Mammilaria, a Melocactus, or a Cereus, it is just as easy to determine the 

 mgitudinal axis of growth in early youth, when these cacti are often as thick as 

 ley are long, as it is later when they are much longer than thick. This is also the 

 ise in the abbreviated axis of bulbs, in many tubers and corms (as the crocus), 

 id in fruits, like those of many gourds, whose diameter is much greater than their 

 !ngth. 



The growth of roots and stems in the direction of their longitudinal axis is 

 generally unlimited, that of leaves and hairs mostly limited, although these rela- 



