. Botany of the living plant. Botany. 168 BOTANY OF THE LIVING PLANT. Fig. 126. Transverse section through the apical bud of Epilobium angiistifolium, L., showing a symmetrical 2 x 2 system. (After Church.) various. It may not be possible to correlate all its forms directly, or even indirectly with circumstance. The difficult question of the actual method, by which such adaptive features as we recognise may have been produced in Descent, must also be left aside. But we may agree to accept as results of adaptation those features which harmonise with the surroundings: and from this point of view
. Botany of the living plant. Botany. 168 BOTANY OF THE LIVING PLANT. Fig. 126. Transverse section through the apical bud of Epilobium angiistifolium, L., showing a symmetrical 2 x 2 system. (After Church.) various. It may not be possible to correlate all its forms directly, or even indirectly with circumstance. The difficult question of the actual method, by which such adaptive features as we recognise may have been produced in Descent, must also be left aside. But we may agree to accept as results of adaptation those features which harmonise with the surroundings: and from this point of view the shoot and its parts may be studied comparatively. An ordinary upright shoot develops as a rule with radial symmetry, that is equally all round the central axis. The axis being cylindrical meets equally the impact of all winds, and its leaves radiate out from it as a centre, occupying a circular area whose radius is the length of the mature leaf. This type is probably a primitive one, and is very general. But it may be worked out variously in detail as regards the arrangement of the leaves, as well as in their form, so as to secure an approximately equal exposure of all the leaves to the incidence of light. It is obviously undesirable that one leaf shall overshadow another, and it is interesting to observe the various ways in which this may be avoided. Following on the paired seed-leaves, the plumular leaves of Dicoty- ledons are often paired also, and at right angles to the first pai/ (decussate arrangement). This arrangement may be maintained through hfe, as it is in the Dead-Nettie, Willow-herb (Fig. 126), Lilac, Sycamore, or Horse-Chestnut. The upright shoot of the Sycamore is a good example how the circular area round the axis is put to the best use by leaves arranged on a decussate plan. Each successive pair fits into the gap between those of the preceding pair. But if the internodes were short, as they are in the young state, the higher would overshadow the next pair
Size: 1859px × 1344px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
