E.OhhO^Y AY .^Six Neiv Zealand Species of Lycopodium. 



363 



to the plane of division (see accompanying diagrams 1-3) ; but this 

 " parallel " arrangement becomes immediately lost in the daughter branches. 

 In the terminal branchlets the number of orthostichies of leaves may be 

 from 4 6 and the number of protoxylem groups in the stele from 3-6, there 



Forking of Stele in Beanchlet of 



L. BiLLARDIERI. 



Forking of Stele in Mature 

 Stem of L. Billardieri. 



being no constant relation between the number of orthostichies and the 

 number of protoxylem groups. The strobili are tetragonous, the leaves 

 being borne in opposite pairs ; the number of protoxylem groups in the 

 strobili is generally three. 



In L. volubile, as the young plant grows the two xylem groups join 

 across, thus separating the phloem into two groups. Next, the stele loses 

 its diarch form, and becomes triarch through the splitting of one of the 

 extremities of the xylem plate. The arrangement is now radial, three 

 xylem plates alternating with three phloem groups. Then, through further 

 splitting the arrangement becomes successively tetrarch and pentarch. 



Development of Stele in Young Plant of L. volubile. 



The individual elements of the xylem are mutually coherent in compact 

 and definite plates radiating from the centre, and the phloem is in groups. 

 This compactness and definiteness in the grouping of the xylem and phloem 

 tissues is a characteristic feature right through the Hfe of the plant. When 

 the pentarch stage is reached, the plates of tissue do not preserve a constant 

 disposition, but are constantly joining up with each other and separating 

 again, presenting various forms of arrangement ; moreover, two protoxylem 

 groups may fuse into one, the pentarch thus passing back again into the 

 tetrarch or even into the triarch. This inabihty of the plates of tissue to 

 preserve strictly constant positions is seen also in the mature stem, although 

 there to a much less degree owing to the parallel arrangement. 



Considerable attention was given to tracing the development of the 

 " parallel " arrangement of the bands of xylem and phloem in the stele 

 of the main stem of L. volubile. Now branching of the 

 stem is frequent, and is always in the plane of the ground. 

 In the young plant, as has been seen, the vascular tissues 

 of the main stem show a radial arrangement, which, how- 

 is constantly changing in conformation. It was 



/.Ti^r 



ever. 



found that at the point of branching of the main stem 

 there is frequently a decided rearrangement of the bands 

 of tissue in a direction at right angles to the plane of divi- 

 sion. In young plants this " parallel " disposition of the 

 bands becomes immediately lost in the resultant branches, but in plants of 

 increasingly greater size it tends to be preserved. 



Forking of Stele 

 IN Young Plant 

 OF L. volubile. 



