048 SECONDARY GROWTH IN THICKNESS 



towards the base. The following measurements may be quoted from 

 Martius : A trunk of the Coconut Palm (Cocos nucifcra), 22'09 m. 

 in length, had a diameter of *46 m. at its upper end, and "74 m. at its 

 base ; in the case of Mauritia Jlexuosa the corresponding measurements, 

 for a trunk 25 - 5 m. in length, were "52 m. and "87 m. In these 

 instances the increase of thickness is usually due to enlargement of 

 the individual cells of the parenchymatous ground- tissue, which 

 thus increases in bulk. The fibrous strands associated with the 

 vascular bundles also augment their cross-sectional area, partly by 

 dilatation of their cell-cavities, and partly by continued thickening 

 of their cell-walls. In Euterpe oleracm, however, according to 

 Kranzlin, the procambial elements which ultimately give rise to 

 bast-fibres, gradually add to their number by division ; the component 

 cells of a given procambial strand are not formed simultaneously, but 

 arise sucessively in centrifugal order. This multiplication of cells in 

 the developing fibrous strands continues until the trunk has attained a 

 diameter of about 1 cm. Strasburger has noted the formation of 

 secondary vascular anastomoses, linking up primary water-conducting 

 strands, in the peripheral part of the trunk of Woshingtonia filifcra. 

 As a rule, however, the vascular tissues are not affected by the 

 " secondary thickening " which a Palm-stem may undergo. 



The above-described very simple type of secondary growth in 

 thickness mainly serves to enhance the inflexibility of the stem ; the 

 tall stature of the trunk renders this increase of mechanical strength 

 very necessary in the case of most Palms. Where secondary thicken- 

 ing depends altogether upon dilatation of the primary tissues, the 

 leaf-output and hence the development of photosynthetic tissue is 

 just as strictly limited, as it is in the entire absence of secondary 

 growth in thickness. For when the paths along which water and 

 minerals travel cannot be continually enlarged, by the addition of new 

 conducting elements, the organs of transpiration and photosynthesis 

 cannot be multiplied indefinitely. In Palms and other long-lived 

 plants with secondary thickening of this primitive type, the leaves are 

 constantly renewed, but their size and number remain constant after a 

 certain stage. 



An unlimited development of the photosynthetic apparatus is, in fact, 

 an impossibility, except where the conducting and mechanical tissues 

 are also capable of indefinite expansion. This condition can only be 

 fulfilled, when secondary growth in thickness consists in a continuous 

 regeneration and amplification of the various tissue-systems of which the 

 stem and roots are composed. In such cases the work of producing 

 new cells is entrusted to a special meristem, which becomes secondarily 

 interpolated among the permanent tissues, and which is termed the 



