282 MR. B. G. COEMACK ON POLTSTELIC 



Scliwendener and his school have shown that, while concentration of the mechanical 

 tissues into a central strand adapts an organ to withstand tensions, a more peripheral 

 position is advantageous in resisting bending ; and Warming and G. Karsten have illus- 

 trated this by showing the correlation between stem-like function and stem-like structure 

 in adventitious roots of Rhizophoracese. 



Thus in roots w^ith cambial developments there are changes amounting almost to a 

 reconstruction on a different plan, together with a redistribution of duties. On the 

 other hand, the Palm-roots in question undergo no such reconstruction, and the plants 

 depend on having their earlier root-system supplemented and replaced by an adventitious 

 system. The older roots are not altered : they are replaced by new and suitable roots. 

 Of this nature are the complex roots at present under consideration. While the root of 

 an oak begins thin and gradually grows thick towards the base, the roots in question 

 begin thick, and grow thin towards the apex. In general there is a correlation between 

 the presence of cambial growth and the absence of polystelic increase ; here there is a 

 bulky primary polystelic development and a gradual return to normal root-structure. 



The figures show that the vascular system lies proportionately much nearer the 

 periphery than it does in a young normal root : with this may be compared the dispo- 

 sition, similar from a mechanical standpoint, attained in other plants through secondary 

 thickening. But the mechanical tissue is disposed centrally as well as peripherally, after 

 a fashion which Schwendener and Haberlandt have shown enables an organ to withstand 

 either flexion or tension. The sclerenchyma-strands previously mentioned are well fitted 

 to resist such tensions. The peculiar disposition of the tissues comes out very clearly in 

 the root of Vei^scltaffeltia previously referred to and illustrated dia grammatically in 

 PI. XX. figs. 14 & 15. While capable of resisting flexion, the structure seems best 

 adapted for resisting tensions as a rope would. The roots seem, on the whole, more of 

 the nature of stays than props ; but at first, before they have firmly fixed themselves in 

 the ground, they must be mainly props. 



As successively younger portions of the root are developed, the mode of differentiation 

 of the meristem continually changes ; the root anticipates its entry on normal environ- 

 ment and duties, and assumes normal root-structure. In this condition there is the 

 relatively thick cortex, more or less sclerenchymatous, and possessing the strong endo- 

 dermis, one of whose functions Schwendener has shown to be mechanical. The main 

 mass of sclerenchyma, however, is the conjunctive tissue of the vascular cylinder, 

 including the large pith. 



Prom the observations thus described it will be seen that certain roots may show the 

 unusual condition of a polystelic structure : further, that in an individual root there is a 

 transition from a polystelic condition, through various stages previously mentioned as 

 types, to normal monostelic form, with concentration of the mechanical tissues into a 

 central strand adapted to withstand the strains to which roots are usually subjected. 



The large radial rifts pre\iously mentioned form an aerating system; their presence 

 and size are to be correlated \a ith the large bulk and relatively small surface of the 

 roots, together with the continuity of the peripheral zone of sclerotic tissue uninter- 

 rupted by such openings as lenticels. 



