776 SUMMARY or CURRENT RESEARCHES RELATING TO 



vascular bundles superposed to the liber-bundles, as metaxylem. In the 

 same manner a metaphloem, as contrasted with the protophloem, is formed 

 contemporaneously with the metaxylem. He sums up his conclusions as 

 follows : — In the primary structure of the root two types may be dis- 

 tinguished : (1) the monoxylic type, where the wood remains in the con- 

 dition of protoxylem ; and (2) the diploxylic type, where the protoxylem is 

 followed by metaxylem. "Where there is any secondary wood, its first 

 vessels are placed opposite the last vessels of the metaxylem, which it con- 

 tinues in a centrifugal direction. 



Formation of the Annual Ring and Growth in Thickness.* — For the 

 purpose of throwing light on the difference in structure between the spring 

 and the autumn wood in the annual ring of dicotyledonous woody plants, 

 Herr A. Wieler has investigated the corresponding phenomenon in a large 

 number of annual plants, and in the annual stems of perennial plants. He 

 finds that in many of these a true annual ring is formed, and very probably 

 in all when the climatic conditions are favourable. This is evidently the 

 case in Heliantlms annuus and Bicinus communis ; and here he believes he 

 was able to demonstrate that the formation of the ring is dependent en- 

 tirely on differences in the supply of nutriment at different periods of the 

 year. 



Autumnal Fall of Leaves, f — Prof. W. Hillhouse proposes to call the 

 layer of cells formed by renewed cell-division in the basal plane of the leaf- 

 stalk, the formation of which causes the dissociation of the leaf, the 

 ahsciss-layer. It is readily recognized by the new dividing-walls formed 

 across the cellular tissue of the base of the leaf-stalk, and by the large 

 quantity of protoplasm contained in the cells, usually accompanied by 

 numerous small starch-grains. It, is usually formed very shortly before 

 the fall of the leaf, which is due to the increased turgidity of the cells of 

 the absciss-layer, owing to their osmotic activity ; they become strongly 

 rounded, their adhesion diminishing at the same time. The soft elements 

 of the vascular bundles are either pinched, or else cell-division takes place 

 in them also ; the lignified elements also undergo changes. 



In leaves about to fall starch is always found in the sieve-tubes, mainly 

 collected in cloudy granular-looking masses in the neighbourhood of the 

 sieve-plates ; this starch stains brown or reddish-brown with iodine. The 

 nucleus, or at least the chromatin, appears to be left behind in the empty 

 cells of fallen leaves, the nucleus tending towards disintegration, as dis- 

 tinguished from fragmentation or direct division. In the leaves of ever- 

 greens examined, starch was also absent in winter, being transferred to the 

 stem, while the tannin, on the other hand, remained behind, as is also the 

 case in fallen leaves. 



(5) Structure of Organs. 



Seedlings of Salicornia herbacea.J — Herr A. Winkler describes the 

 structure of the seedlings of this plant, which possess the peculiarity of 

 the two cotyledons being coalescent at their base, the cone of growth lying 

 in a depression between them. 



Formation of Rootlets and position of Buds in the Binary Roots of 

 Phanerogams. § — M. P. van Tieghem states that the place of formation 

 of rootlets in the pericycle of the mother-root is fixed by two rules, and not 



* Pringsheim's Jahrb. f. Wiss. Bot., xviii. (1887) pp. 70-132 (2 pis.), 

 t Eep. Brit. Assoc. Birmingham Meeting, 1886, pp. 700-1. 

 J Verhandl. Bot. Ver. Prov. Brandenburg, xxviii. (1887) pp. 32-3. 

 § Bull. Soc. Bot. France, xxxiv. (1887) pp. 11-6 and 39-44. 



