ISOilTES JAPONICA, A. BE. 



341 



The factors wMcli bring about this result were investigated by Farmer (14), and we 

 cannot do better than quote his explanation, which is entirely confirmed by our own 

 investigations. Parmer (14, p. 41) states that "This result is indirectly brought about 

 by the activity of the cambium. The traces of the roots and leaves which pass through 

 this zone to join the axile vascular cylinder are subjected to great strain by the fact 

 that, whilst the cells round them are growing, and, so to speak, travelling outwards, they 

 themselves are stationary, and can only increase in length by extension. This takes 

 place to a limited extent, and effects a great distortion of their constituent cells, but 

 finally the loose central tissue gives way, and they are partially relieved of the strain 

 at the expense of the cohesion of the inner tissue." Moreover, this explanation accounts 

 for the fact that the pull exerted is in the horizontal direction. In this connection tlie 

 ' parenchymatous mantle ' probably plays an important part. As mentioned above, the 

 primary xylem is completely enveloped by the 'parenchymatous mantle,' which is 

 uninterrupted except by the leaf -traces. The parenchyma of the leaf-traces is connected 

 up with that of the ' parenchymatous mantle,' so that as the leaf -traces are pulled away 

 from the central axis of the stem they drag the ' mantle ' with them, with the result 

 that the weaker internal tissues of the stele are compelled to give way. The presence 

 of this parenchymatous mantle also explains the regular outline of the primary tissues 

 of the stele of an adult plant as shown in a median longitudinal section of the stem ; 

 if the leaf-traces were connected up directly wdth the loose internal xylem-parenchyma 

 or with the tracheids of the stem-stele, the outline of the latter would be very 

 irregular {of. text-figs. 1, 2; PI. 33. fig. 8; PI. 34. fig. 10). 



1 



2. Primary Thloem. 



+ 



The sieve-tubes of the primary phloem have thick cellulose walls, which, in well- 

 stained preparations, show typical sieve-areas. In many of these primary sieve-tubes 

 deposits of callus were observed, and in some cases the deposit was so abundant as to 

 completely fill the cavity of the cell (PI. 34. fig. 13 ; PI. 38. fig. 64). 



Primary phloem of a similar character was also found in Isoetes lacustris (PI. 38. 



fig. 62). 



The sieve-tubes, which are continuous with those of the leaf-traces (PI. 34. fig. 13 ; 

 PI. 35. fig. 15), are associated with unmodified cells of the 'mantle,' several of which 

 contain a semi-transparent substance, which is remarkably resistant to stains. Possibly 

 these parenchymatous cells have a function similar to that of the companion cells of 



the sieve-tubes of Phanerogams. 



The cells of the primary phloem are seldom, arranged in regular radial rows, and it 

 is obvious from their arrangement that they cannot have been produced by the activity 

 of the secondary cambium. They are often elongated tangentially (PI. 34. fig. 13), 

 being subjected to the same horizontal pull as that mentioned above in connection with 

 the primary xylem. The primary phloem of the stem would serve to connect up the 

 phloem of the first-formed leaf-traces with the corresponding tissue of the earlier roots, 

 prior to the formation of the secondary phloem. 



