Dar bishire. — Observations on Mamillaria elongata. 401 
The arrangement of the rows of palisade-cells offers a strong confirma- 
tion of the view held by Stahl, that the characteristic development of this 
tissue is influenced by the strength of the light ( 30 , pp. 36-3 8). 
Haberlandt sees in the cell-rows of the palisade-tissue an arrangement 
for rapidly conducting away the elaborated organic products of photosyn- 
thetical activity. Their position and direction is in no way influenced by the 
light directly ( 13 , p. 250). Eberdt puts down the varying development, 
but not the presence of palisade-tissue, as an adaptation to give transpiration 
and photosynthesis equal rights ( 19 , pp. 373, 374). It is in fact a com- 
promise between the two. Personally I should consider the elongation of 
the cells to be a concession only to photosynthesis, but the reduction in 
depth of the air-spaces accompanying the palisade-tissue as a concession to 
transpiration. 
Nordhausen has recently published some very interesting observations, 
which in one particular are, however, not yet quite complete. He shows that 
the buds of Beech shoots, which have been grown in the shade, give rise to 
shade-leaves even when grown exposed to the full light (21, pp. 30 to 45). 
The bud in fact takes on the character of the leaf, in the axil of which it 
is formed. The chances generally are of course that it will grow up 
surrounded by the same conditions as that leaf. This is an interesting 
fact. But, as Nordhausen himself points out, it is desirable that further 
experiments should be carried out. A bud is developed in the axil of the 
leaf, which though itself exposed to strong sunlight is derived from the bud 
of a shade-leaf, and shows shade-leaf characters. Does this last bud 
produce shade-leaves or sun-leaves at once, or only after one or two years ? 
The removal of the branch with its leaves from shade to light before the 
leaves in the bud are quite differentiated might lead to interesting results, 
and show how direct the influence of shadow and light is. 
Areschoug (1, pp. 1-18, 38-43) lays greatest stress on the influence 
which the external conditions exert on the plant in its desire to keep the 
transpiration-stream under control. The reduction of air-spaces causes 
a reduction in the rate of transpiration. Therefore the palisade-tissue with 
its not very extensive air-spaces is an adaptation to reduce transpiration. 
In the case of Mamillaria elongata and those Cactaceae which show 
similarly equipped tubercles the palisade-tissue is clearly in my opinion 
a protection against the influence of the strong light on the green plastids, 
and not against undue transpiration, for if the latter were the case the inner 
cells would also show a similar arrangement (PI. XXV, Fig. 14). The air- 
spaces in the inner tissue of the tubercle are slightly bigger than those in 
the palisade-tissue, but the cells are also less exposed to strong sunlight. 
The extensive air-spaces by which almost every one of the carbon- 
dioxide absorbing cells is bordered to a smaller or larger extent do not 
separate the cell-walls very much. They are, that is to say, long but 
