STRUCTURE OF THE ROOT 



85 



of phloem are not strongly developed, and the whole is compacted 

 by cells of the conjunctive parenchyma, which fill up the interspaces 

 and extend to the centre. Here its cells instead of forming a soft 

 pith become sclerotic with age, so that the lignified tissues are all 

 welded together into a central, mechanically resistant strand. 



Internally to the phloem cell-divisions may be seen at several points in the 

 Fig. 58. These are in the position where in other roots a cambial activity 

 arises. Here, however, as in Monocotyledons generally, the divisions pro- 

 ceed nc further. It will also be noticed that the intercellular spaces in 

 the cortex are large. Acorus is a swamp-growing plant, and the tissues of 

 water-plants are characterised by large intercellular spaces. 



The arrangement of the vascular tissues thus seen in roots, with 

 the xylem and phloem alternating on different radii, is described as 



Fig. 59. 



Transverse section of the stele ot a young root of Ranunculus, showing the central 

 metaxylem not yet developed. Lettering as before. ( x 200.) 



radial. It is in sharp contrast to the collateral arrangement charac- 

 teristic of stems, where the xylem and phloem are upon the same 

 radius, the phloem being outermost. Moreover, while in stems the 

 protoxylem is directed centrally, in roots it is peripheral in position. 

 Evidence of the centripetal succession of development can easily be 

 seen in sections of young roots. Fig. 59 shows such a section from 



