812 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
dependent on the vital properties of the protoplasm ; capillarity is 
of itself insufficient to explain the phenomenon, and transpiration is 
important only in so far as it makes room for the ascending water. 
The conditions necessary for the ascent of water are (1) that the cell- 
walls should be in a state of imbibition ; (2) that the cavities of the 
tracheae should be to a certain extent filled with water ; and (3) that 
they should be isolated so as to exclude the entrance of air. Atmo- 
spheric pressure helps to keep the water suspended, but does not cause 
its ascent ; and root-pressure is not immediately concerned in the 
process. 
Appearance of the first Vessels in the Flowers in Lactuca.* — 
According to M. A. Trecul, in Lactuca oleifera and L. perennis the first 
vascular cells make their appearance only in the lobes of the corolla ; 
while in L. sativa they appear both in the corolla-lobes and in the fila- 
ments ; and in L. virosa in the filaments only. The first vessels of 
the stigmatic branches appear, in L. sativa , oleifera , and virosa, after 
those of the corolla and of the filaments ; and in L. perennis, sometimes 
later than the first vessel of the ovary. Subsequently these vessels of 
the stigmatic branches, reinforced by numerous vascular cells, give rise 
to a bundle which descends into the style. The author further traces 
the order of appearance of the first vessels in the ovary and in the 
pedicel, which presents certain differences in the different species. 
Comparative Structure of Woods.f — Herr Pommerenke notes 
several peculiarities in the wood of various trees belonging to sympe- 
talous families from Argentina. He finds conjugating cells to be com- 
mon in the medullary rays and the xylem-parenchyme, not only of the 
Ebenacese, but of other natural orders. Conjugation appears to 
depend on the occurrence of medullary rays consisting of only a single 
row of cells. These are frequently pinched in and compressed between 
elements which are often libriform fibres, but sometimes vessels or cells 
of the xylem-parenchyme. Between the medullary cells which are 
thus separated, delicate connecting tubes can be detected. A conjuga- 
tion may also take place between dissimilar elements. Branched libri- 
form fibres and tracheids were observed in several cases. The occur- 
rence of patches of pith in the xylem was also noted. 
Causes of Variation in the Density of Wood.J — According to 
M. E. Mer the density of wood depends on two factors, on the relation 
between the size of the cells and the thickness of their walls, and on 
the constitution of the walls and the extent to which they are lignified 
or impregnated by tannin and resin. The density, therefore, varies 
not only with the conditions in which the tree grows, but also in 
different parts of the same tree. In the inner or spring portion of a 
zone the cells are usually larger and have thinner walls than in the 
outer or summer portion of the same annual zone. When the activity 
of the cambium is greatest, cells of large size are formed rapidly, but 
the plastic materials are not present in sufficient quantity for the walls 
to acquire any great thickness, while the reverse is the case when the 
* Comptes Rendus, cxv. (1892) pp. 8 5-92. 
+ JB. Schles. Gesell. Vaterl. CuJtur, lxix. (1892) pp. 85-6. 
X Bull. Soc. Bot. France, xxxix. (1892) pp. 95-105. Cf. this Journal, ante, p. 511. 
