The principles of biology . aths, may be accompanied bydifferent sets of modifications. Supposing Fig. 81 to be atransverse section of such a type [a being the mid-rib, andb the expansion of an older frond; while c is a yoimger frondproHferously developed within it), there may begin two di-vergent kinds of changes, leading to two contrasted struc-tures. If, while frond continues to grow put of frond, theseries of united mid-ribs continues to be the channel of circu-lation between the uppermost fronds and the roots—if, as aconsequence, the compoimd mid-rib, or rudimentary axis, con-tinues to in
The principles of biology . aths, may be accompanied bydifferent sets of modifications. Supposing Fig. 81 to be atransverse section of such a type [a being the mid-rib, andb the expansion of an older frond; while c is a yoimger frondproHferously developed within it), there may begin two di-vergent kinds of changes, leading to two contrasted struc-tures. If, while frond continues to grow put of frond, theseries of united mid-ribs continues to be the channel of circu-lation between the uppermost fronds and the roots—if, as aconsequence, the compoimd mid-rib, or rudimentary axis, con-tinues to increase in size laterally; there will arise the seriesof transitional forms represented by the transverse sections82, 83, 84, 85; ending in the production of a soHd axis,everywhere wrapped round by the foliar surface of thefrond, as an outer layer or sheath. But if, on the other 56 MORPHOLOGICAL DEVELOPMENT. hand, circumstances favcur a form of plant which maintainsits uprightness at the smallest cost of substance—if the. 89 vascular bundles of each succeeding mid-rib, instead of re-maining concentrated, become distributed all round the tubeformed by the infolded frond; then the structure eventuallyreached, through the transitional forms 86, 87, 88, 89, willbe a hollow cylinder. And now observe how the two structures thus produced, correspond with two kindsof Endogens. Fig. 90 represents a species of Dendrohium,in which we see clearly how each leaf is but a continuationof the external layer of a soHd axis—a sheath such as wouldresult from the infolded edges of a frond becoming adnate;and on examining how the sheath of each leaf includes theone above it, and how the successive sheaths include the axis,it wiU be manifest that the relations of parts are just suchas esat in the united series of fronds shown in Fig. 79—^thesuccessive nodes answering to the successive points of originof the fronds. Conversely, the stem of a grass, Fig. 91, dis-plays just such relations of part
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectbiology, bookyear1864
