1 68 



BOTANY OF THE LIVING PLANT 



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 pair 

 (decussate arrangement). This arrangement may be maintained 

 through life, 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 but one of lower leaves. This 

 difficulty is met by the lower pair having longer petioles, so that their 

 blades are carried out beyond those of the leaves immediately above 

 them, forming a compact " leaf -mosaic " (Fig. 127). 



The decussate is the simplest of the cyclic or whorled arrangements, 

 where two or more leaves are seated at the same level. But in 

 other cases the number of the leaves at the same level may be 

 not two only, but three, four, or more. As in the decussate plan the 



FIG. 126. 



Transverse section through the apical bud 

 of Epilobium angustifolium, L., showing a 

 symmetrical 2x2 system. (After Church.) 



