TRANSACTIONS OF SECTION K. 759 



Summer. — Species of Mougeotia with desmids. 



In one shallow pond with deep mud Spiroyyra longata (Vauch.) Kutz. domi- 

 nates throughout the year, but in the hotter months of 1905 this was invaded by 

 Microcystis roseo-persicinvs, Kutz. which formed a striking pink 'water blossom.' 

 Phormidium iiiundat/on, Kutz., also occurred as a covering upon the mud. During 

 hot sunny days this alga rises to the surface, and there becomes disposed in a thin 

 layer. This ascent to the surface is due to the motility of the filaments, and takes 

 place along threads of Spirogyra, which often form the core of the ascending 

 column. This power of movement is of great importance to species of Oscillatoria 

 and Phormidium, and enables them to migrate from a situation of extreme stagna- 

 tion to the better aerated surface region. 



Site 3. — Short decaying shoots of the smaller flowering plants, such as Carex 

 vulgaris and Eleocharis palustris, which cover the floor of the pond in the shallower 

 parts not occupied by ffinanthe. Dipterous insect larvae, nais and nematoid 

 worms, abound here and influence the algal vegetation. The dominant alga is 

 Oloeocystis vesiculosa, Nag. Many other small gelatinous forms flourish in this 

 situation. 



TUESDAY, AUGUST 7. 



The following Papers were read : — 



1, On the Seedling Structtire of Gymnosperms. 

 By T. G. Hill and E. de Fraine. 



Ta.vtis baccata. — The two cotyledons each contain a single collateral bundle, 

 which is, in some cases, slightly mesarch. A section of the axis, taken through 

 the cotyledonary node, shows the central region to be occupied by six plumular 

 bundles arranged in two groups. Each seed-leaf trace, at its entry into the 

 hypocotyl, has its xylem an-anged in a V-li^e manner, the protoxylem being near 

 the apex, which is pointed outwards. Each cotyledonary bundle travels obliquely 

 downwards, and during its passage the phloem is resolved into two parts. At 

 the same time the metaxylem passes in more quickly than the protoxylem, so that 

 the latter, for a time, is entirely mesarch in position. The phloem masses of the 

 cotyledon bundle fuse with the corresponding tissue of the nearest plumular 

 bundle — the three strands of each group having fused together — and, con- 

 currently, the metaxylem of the same traces moves towards the xylem of the 

 epicotyledonary bundles ; hence the protoxylems of the seed-leaves are left in a 

 more exarch position, and, ultimately, are quite exposed. The vascular tissues 

 close up and become more compact, thus a typical diarch root results, [t is not 

 possible to distinguish any rotation of the protoxylem : it ultimately occupies an 

 exarch position, because it is left isolated by the movements of the metaxylem. 

 There may be made out, however, an indefinite rearrangement of the protoxylem, 

 coupled with a slight centrifugal movement, after the exarch position has been 

 attained. 



Ta.vus cuspidata, as far as can be ascertained from the study of old seedlings, 

 does not ditt'er in any essential feature from the above species. 



Cephalota.vus pedunculata is very similar to Ta.vus. The main points of 

 difference are as follows: (a) The single bundle of each cotyledon shows the 

 mesarch structure more highly developed ; and {h) the seed-leaf traces do not 

 bifurcate so soon. They enter the central region of the axis as well-defined 

 collateral endarch structures, and form a cylinder with the plumular traces, the 

 phloem forming a practically closed ring. At a lower level, the bast-circle 

 divides, opposite the cotyledon bundles, into two well-defined masses. During 

 this rearrangement the xylem groups come into contact, the metaxylem derived 

 from the seed-leaves joins up with the plumular xylem, and thus the pi-otoxylem 

 becomes isolated. The subsequent changes are essentially the same as those 

 described for Taxus, 



