50 



MORPIIOLOG ICAL DEV ELOPMENT. 



hand, circumstances favour a form of plant which, maintains 

 its uprightness at the smallest cost of substance — if the 



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Si 



vascular bundles of each succeeding mid-rih, instead of re- 

 maining concentrated, become distributed all round the tube 

 formed by the infolded frond ; then the structure eventually 

 reached, through the transitional forms 86, 87, 88, 89, will 

 be a hollow cylinder. And now observe how the 



two structures thus produced, correspond with two kinds 

 of Endogens. Fig. 90 represents a species of Dendrobvum, 

 in which we see clearly how each leaf is but a continuation 

 of the external layer of a solid axis — a sheath such as would 

 result from the infolded edges of a frond becoming adnate ; 

 and on examining how the sheath of each leaf includes the 

 one above it, and how the successive sheaths include the axis, 

 it will be manifest that the relations of parts are just such 

 as exist in the united series of fronds shown in Fig. 79 — the 

 successive nodes answering to the successive points of origin 

 of the fronds. Conversely, the stem of a grass, Fig. 91, dis- 

 plays just such relations of parts, as would result from the de- 

 velopment of the type shown in Fig. 79, if instead of the mid- 

 ribs thickening into a solid axis, the matter composing them 

 became evenly distributed round the foliar surfaces, at the 



