

Manchester Memoirs, Vol. lix. (19 1 5), No. 3. 25 



the roots, outwards, until exposed at the split, and con- 

 tinues to effect the progressive carriage of the older roots 

 away from the groove. It is consistent with this explana- 

 tion that there is no penetration of the cortex by the 

 roots themselves. These, as exposed at the groove, are 

 seen to stand as practically superficial and exogenous 

 structures, with projections of the free cortical surface 

 between their bases (Fig. 5). The root tip, though 

 developed close to the growing line, has been passively 

 carried out by the radial extension of the cortex. Thus 

 the position of the roots, the direction of the young root- 

 tip to the plane of the split and the curve of the root- 

 traces all find a natural explanation in this way of looking 

 at the growth of the rhizophore, while they are meaning- 

 less on any other view. It is also clear that the nature of 

 the development involves the recognition of a root-bearing 

 region of the cortex distinct from the leaf-bearing cortex 

 of the shoot and indications of this distinction have been 

 noted in all planes of section through the stock. 



The growth of the upper and lower regions of the 

 stock of Isoetes, though separately considered in the pre- 

 ceding paragraphs, is, of course, correlated in the plant as 

 a whole. The progressive cortical increase, resulting in 

 the cortical lobes, stretches and obliterates both the older 

 leaf-traces and root-traces. The correlation between the 

 growth of the shoot-cortex and rhizophoric cortex will be 

 evident on consideration of the median section of a com- 

 plete plant in the plane of the lobes in Fig. 5, 



Having now seen how the view that the stock of 

 Isoetes is composed of a shoot and a rhizophore affords 

 a satisfactory and connected explanation of the chief 

 peculiarities of structure and growth of the individual 

 plant, we may in the second place briefly enquire how 

 this view lends itself to comparison with other plants. 



