TRANSACTIONS OF SECTION K. 333 



the medullated stele of Platyzoma may have been derived from a protoatelic origin 

 by the dii"ect formation of an independent intrastelar pith and inner endodermis. 



The evidence so far advanced seems inadequate to allow of a confident opinion 

 regarding the nature and origin of the medullated stele of Platyzoma. For while 

 no evidence of the intrastelar origin of the pith and inner endodermis has emerged 

 to support the latter interpretation, the reduction theory has not been supported 

 by the demonstration of cortical ' intrusion ' into the stele through stelar gaps, nor 

 by the presence of stelar gaps and a solenostelic condition. 



A recent investigation of a number of specimens of Platyzoma showed neither 

 a solenostelic condition nor evidence of stelar gaps or inner phloem. On the 

 reduction hypothesis they might be considered evidential of the firm establishment 

 of the reduced condition. On the theory of upgrade intrastelar differentiation 

 they might be held to demonstrate how entirely intrastelar in nature as in location 

 are the pith and inner endodermis. 



On the other hand, as the stele of a small but incomplete specimen was followed 

 forward from the broken base of the stem the pith and inner endodermis decreased 

 until the latter was narrowed to a vanishing point, so that a medullated protoetele 

 was established locally. The pith again expanded and within it there arose 

 de novo an inner endodermis. Further forward the pith diminished and the inner 

 endodermis again narrowed to a vanishing point. For at least the second time 

 in the history of this plant a medullated protostele was established locally, and 

 on subsequent re-expansion of the pith, inner endodermis was again created 

 de novo, and was subsequently maintained to the apex of the stem as an unbroken 

 cylinder. 



It is thus recognised that Platyzoma may become locally protostelic by reduc- 

 tion of the pith and loss of the inner endodermis. It may augment from within 

 the stele itself an attenuated pith. In like manner it may develop an independent 

 intrastelar endodermis. 



Three definite states may exist in the stele of a fern. 



(i) The original protostely may persist undisturbed throughout the entire 

 ontogeny, 



(ii) The original protostely may be transformed at an early stage in the 

 ontogeny once and for all into one or other of the more elaborate stelar states. 



(iii) The original protostely may recur in the ontogeny in ferns in which the 

 advanced stelar state is not yet permanently established. 



It is now suggested that Platyzoma provides an illustration of the third and 

 apparently exceptional state in which the protostelic structure of the ancestry 

 has not yet been definitely restricted to the 'sporeling' stages, but tends to 

 persist in the mature organism. The stelar structure of the small specimen may 

 be reminiscent of the steps taken in the initial transformation of the ancestral 

 protostele, namely, the growth of a pith within the stele itself, and the inclusion 

 of the bulk of this intrastelar pith within an independent internal endodermis 

 created de novo. On this view tbe present medullated conductive cylinder of 

 Platyzoma would be regarded as the high-water mark of stelar amplification so 

 far reached for this plant, neither inner phloem nor stelar gaps having been 

 evolved. 



5. The Northern Invasions of New Zealand, with Special Reference to 

 Lord Howe Island. By J. 0. Willis, Sc.D., F.R.S. 



Detailed study of the local distribution of species in New Zealand by zones 

 of 100 miles in width reveals the probability that, in addition to the invasions 

 at the north, from the Kermadecs, and from the south there was another 

 invasion from the west, which likely followed the ridge upon which stands 

 Lord Howe Island. By the usual method of prediction and subsequent verification 

 it is then shown that the Lord Howe flora consists mainly of the genera of the 

 three northern and western invasions ; that the endemics of its flora are mainly 

 in the older families of these invasions, and in the larger (older) families and 

 genera of Howe; and that a large proportion of Howe species reach the 

 Chathams, which are opposite to the probable centre of the western invasion 

 of New Zealand. 



