66 RELATIONSHIPS OF SYMMETRY 



plants that the lateral twigs are the more developed' the nearer they lie 

 to the point of the annual shoot, an arrangement which appears specially 

 to favour a regular expansion of the woody skeleton. A pine-tree has 

 no lateral shoots at the base of its annual shoot ; higher up spur-shoots 

 appear upon this, but only at the summit do the long shoots arise in 

 a false whorl ; there is produced in this way upon the chief shoot itself 

 a tiered structure whereby the branches do not cover one another, 

 whilst on the lateral shoots the branches of higher order always arise 

 further from the chief stem, and so carry the organs of assimilation to the 

 periphery where they will find the most favourable illumination. Many 

 broad-leaved trees behave in a similar manner, only a more gradual 

 gradation takes place in them, and the buds which are found at the 

 base of the annual shoot are frequently destined to act as resting buds 

 which only unfold in the event of injuries to the plant making a call 

 upon them. 



Apex and base of a plant or of its part may be connected by an 

 imaginary line which we designate the long axis. 



Leaving out of consideration a few exceptional cases, we may 

 distinguish in the arrangement of the lateral organs and the construction 

 of the organs themselves three kinds of cases : 



1. Radial construction. This it is when an organ shows no differen- 

 tiation into an anterior and a posterior side, nor into a right and a left 

 side, but is organized about the long axis in every radius of the transverse 

 section in nearly the same manner J . 



2. Bisymmetric or bilateral organs. We understand, by these, organs 

 which have an anterior and a posterior side, and a right and a left side, 

 which are respectively like to one another. The distichously-leaved 

 shoot of Schistostega (Fig. 25) and Fissidens, and the pinnate thallus 

 of Bryopsis, are, for example, bilateral. In Schistostega the bilateral 

 configuration has moreover arisen in the course of development out of 

 a radial one. The leaves, which in the mature shoot stand in two rows 

 attached throughout their length to the stem, are inserted transversely 

 on the vegetative point and distributed around the shoot-axis (Fig. 26). 

 Opuntia shows this transition even more simply: the radial shoot-axis 

 becomes flattened on two opposite sides and thus develops into a bilateral 

 structure. How near radial structure stands to bilateral structure we 

 also see in many marine Algae, of which the thallus, fixed only at its 

 base, floats freely in the water and is sometimes flat, that is bilateral, 



1 This form of construction was originally designated the ' concentric ' by E. Meyer in Linnaea, 

 vii. p. 149 a term which has rightly been passed over. It was especially unsuited to the radial 

 distribution of lateral organs. Unfortunately A. Braun introduced a special terminology for the 

 flower ; radial flowers he designated ' actinomorphous/ dorsiventral he termed ' zygomorphous.' 

 These clumsy names are in my opinion altogether superfluous. 



