1 8a EXTERNAL CONFORMATION OF PLANTS. 



Sect. 27. Directions of Growth ^ — (i) In every thallus, branch, stem, leaf, 

 hair, and root, it is easy to distinguish between two opposite ends, the Base and the 

 Apex. The base is the place where the member was first formed and began to 

 grow; the apex Hes in the direction which the growth follows. The direction 

 from the base to the apex is the longitudinal direction of the member under 

 consideration ; 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 the Organic Centre of the transverse section. Every line drawn from this point 

 towards any point of the circumference is a radius ; every definite 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 distinguished from them. These relationships are recognised 

 with ease in the transverse section of woody stems and roots, but can be easily made 

 out in other cases also, even in unicellular plants and hairs. The organic centre of 

 the transverse section does not usually coincide with the geometrical centre, as is 

 easily seen in the transverse sections of most leaf- stalks 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 when further deve- 

 loped (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, the latter 

 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 shown 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 questionable 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 Isoetes, 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 

 ind apex of a member ; and its position on the transverse section (the organic 

 :entre) can be found without anything else being known about the relationships of 

 growth. It is always possible, without even knowing the duration or intensity of 

 ;he growth, to decide which is the longitudinal and which the transverse section of a 



* H. von Mohl, Ueber die Symmetrie der Pflanzen, in his Vermischte Schriften. 1846. — 

 iVichura, Flora, pp. i6i et seq. 1844. — Hofmeistdr, Allgemeine Morphologic, §§ i, 23, 24. Leipzig 

 1868. — Pfeffer, Arbeiten des botan. Instituts in Wiirzburg, p. 77. 1871. 



