190 Transactions. 



clearly marked off from the remainder. These characters are — a plagio- 

 tropic habit of growth with all-round branching of the trailing stem ; a 

 mixed type of stelar anatomy with exceedingly broad protoxylem groups ; 

 a delicate short-lived surface-growing prothallus ; and a protocorm stage 

 in the embryogeny. It may safely be assumed that a type of prothallus 

 which possesses chlorophyll and is largely self-nourishing is less modified 

 from the original type than are those which are wholly saprojihytic in mode 

 of nutrition. This assumption is made by most writers on the subject. It 

 is not, however, necessary to assume that the delicate nature of the cernuum 

 type of prothallus is also a primitive character. Such a type of prothallus 

 demands a damp habitat. Another consecjuence of the delicate nature of the 

 prothallus is that it becomes necessary for the young plant to quickly gain 

 independence and establish itself, and hence probably arose the protocorm 

 as a physiological specialization. Such damp situations as are suitable for 

 the development of this type of prothallus are liable to seasonal periods of 

 drought, and hence the protocorm is further developed as a resting tuber. 

 The mixed type of stelar anatomy is initiated by the fact that in the very 

 young plant the leaf-trace system precedes the formation of the cauline 

 cylinder, and thus the stelar tissues show from the first a loosely aggregated 

 character. Moreover, in the older stems the branching of the stele in all 

 four directions tends to continually disturb the tissues. However, this is 

 not sufficient to account for all the peculiar features in this t}'pe of stele, 

 which would seem rather to be an expression of the inherited constitution 

 of this division of the genus. The plagiotropic habit and unlimited growth 

 of the plant are a modification suited to assist it to spread over wide areas, 

 and are able to counterbalance as means of vegetative propagation the 

 uncertainties attached to the sexual reproduction of these species. Thus all 

 these characters are largely dependent upon the environment, being under 

 its direct influence, and with respect to them the different species show a 

 high degree of plasticity. 



Species belonging to the Clavata Section. 



I have already enumerated certain reasons for removing the species 

 L. densum, L. volubile, and possibly also L. obscurum and L. casuarinoides, 

 from Pritzel's Cernua section, and placing them instead in the Clavata 

 section, and for removing the species L. dijfusum and L. ramulosum from 

 Pritzel's Clavata section and placing them in the Cernua section. Such a 

 rearrangement of these species would seem to be in accord with the facts 

 known concerning their main gametophytic and sporophytic characters. 



This section is marked by a number of very definite characters. The 

 habit of growth of the species is strongly plagiotropic — indeed, so much, 

 so that several show a bilateral structure in the leaf-arrangement, with 

 heterophylly, and, at least in the New Zealand species, the branching is 

 always in the plane of the ground, and not all round the stem as in the 

 species of the last section. The fertile regions are upright club-shaped 

 cones borne on distinct pedicels, or numerous short cones at the tips of 

 the densely-branched aerial branches, except in the case of L. volubile, 

 where the scrambling habit of the plant has probably been accountable 

 for a noteworthy modification. The stelar anatomy shows a characteristic 

 dorsiventral disposition of parallel plates of xylem and phloem. The 

 prothallus is large, compact, and deeply buried, and the young plant pos- 

 sesses a strongly developed foot. These characters clearly mark off the 

 Clavata section from the other sections of the genus. It is, of course, obvious 



