BOTANY OF THE LIVING PLANT 



m which follow closely the surface to which they become attached. 



nally they may even become green, and act as effective organs 



Photosynthesis. But these are exceptional cases. Speaking 



ncrally the root of Flowering Plants retains its uniform cylindrical 



outline and the whole root-system is built up as regularly or radially 



.metrical. This fact may rightly be related to the uniformity of its 



usual surroundings. 



"I lie Shoot, on the other hand, is exposed to much more varied con- 

 ditions than the root. It may be developed in water, still or moving ; 

 or if developed in air, it may be subjected to various degrees of lighting 

 and moisture, and to winds from any quarter, as well as to the various 

 incidence of gravity. It is possible to trace, in the different forms of 

 the shoot which we see,, a relation to and fitness for its surroundings. 

 It would be strange if the shoot, which is so adaptable individually 

 as we have seen it to be (Chapter IX.), should not show variety 

 of conformation in the race, seeing that its surroundings are so 

 diverse. 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 be also left aside. But we may 

 agree to accept as results of adaptation those features which harmonise 

 ^^ y with the surroundings : and from 



W si /f^K \ >w 1 tn * s P omt °f v * ew tne snoot anc * its 

 ^f I /[ x^x/^X P arts ma Y b e studied comparatively. 

 // / sf VxVv 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 prior pair 



Fig. 133. 



Transverse section through the apical bud 

 of Eptlobium angustifolium, L., showing a 

 symmetrical^ ■• 2, or decussate system. (After 

 Church.) 



