THE AXIS OF GROWTH. 483 



which contains the axis of growth is an axial longitudinal section. If the axis is 

 curved in one plane there is of course only one axial longitudinal section ; if, how- 

 ever, the organ is itself spiral, the axis of growth also describes a spiral line which 

 cannot be contained in one plane. 



As a rule the growth in the direction of the longitudinal axis is more vigorous — i.e. 

 is more rapid, or continues longer, or both together — than is the case in directions across 

 the axis, as is at once seen in the case of most shoot-axes, roots, leaves, and hairs. 

 This character, however, cannot be employed for the general definition of the axis of 

 growth : there are cases where the growth takes place more energetically transversely 

 to the longitudinal axis than parallel with it, as, for example, in the stem of Isoe/es, in 

 the short and broad fruits of some varieties of the Gourd, and in many leaves which 

 are broader than they are long. However, no doubt can ever arise with respect 

 to the axis of growth in such cases, since it is always possible to determine with 

 certainty whether the transverse or the longitudinal section of an organ is under 

 observation, and with a little practice even 

 a very small portion of a transverse or 

 longitudinal section suffices for the re- 

 cognition of its true nature. 



If the growth in the direction of the 

 axis is unlimited in the sense given above, 

 the formation of external organs as well 

 as the internal structural relations repeat 

 themselves in the same direction ; the 

 organ becomes segmented into a series 



r>f nnrtc vuViiVVi mciv Ka rlictinmiicVif^rl cia Mp- '""'■■ 319.— Anterior portion of a shoot o^ Herposiphoma 



01 pans wnicn may oe aistmguisnea as me- ,.^^„,^ („„^ „r the Fiondea). Froir, the shoot-axis, which is 



tTTTiprAC cin AvnrAcoir^n olrf>irK' ^^mr^lnv^rl curved forwards and upwards, and the g:rowing-point of which 



lamereS an expression aireaa)- empiOyea j^ ^j „^ j^e leaves b'-bs arise ; these are likewise curved. 



in Zoology. The individual internodes -/ '-'-1 shoot ;.. root (after coebei). 

 of a shoot, with the leaves or whorls of 



leaves, lateral buds and other organs belonging to them, represent these metameres. 

 If, however, the free growing-point of a shoot or leaf normally concludes with the 

 formation of some special organ, metameres may still be present, but they are not 

 similar to one another; on the contrary, they undergo metamorphosis from the 

 base towards the apex, as is particularly evident, for example, in the different forms 

 of the foliage leaves on upright shoot-axes which conclude with a terminal flower 

 (^Ranunaihis acer, Papaver somniferuni). The shoots of many horizontal creeping 

 or climbing plants, on the other hand, consist of metameres which resemble one 

 another, e.g. Lysimachia nummidaria, Cucurbita, Glechoma, Marstlia, etc. 



The presence of an axis of growth does not necessarily imply a distinction of 

 base and apex also, although other cases may occur only among the very lower 

 Algse. For instance, the filaments of the Algae Spirogyra and Sphccroplea, as well 

 as the filamentous cell-families of the Desmidiese and Diatomece, have always a 

 longitudinal axis, though the two ends of the filament exhibit no distinction of apex 

 and base. With this state of affairs is connected the fact that such plants are not fixed 

 by the end to a substratum. In the globular cell-families of the Volvocinese we find 

 no distinct axis of growth at all, but only a middle point around which the whole 

 organisation is more or less symmetrically arranged on all sides ; and some flat discoid 



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