Senecio precox, D.C., from Mexico. 35 
and receptacles from which exude a resinous or balsamic sub- 
stance. While botanizing in Mexico during the summer of 
1896, plants in all stages of growth were gathered and pickled 
in a barrel containing one per cent formalin.* 
Locking at a cross section of a stem at least an inch anda 
quarter in diameter, which has been preserved in formalin, one 
is struck by a small disc-like area in the centre of the pith, a 
trifle over a quarter of an inch in diameter. In longitudinal 
section, the depressed circular area is seen to correspond to 
lens-shaped spaces, between the watery lamellz or discs of 
pith, one-eighth of an inch or more in thickness (Plate VIII, 
Fig. 1). The pith cells are large, nearly twice as long as 
broad, and are filled with water (which is evidently cellular 
water), because in the formalin material, the protoplasm was 
found balled in the centre of the cell. The discs of pith are 
highly turgescent in the living plants (Plate VIII, Fig. 1), and 
a considerable amount of water is thus stored up, whether in 
combination with a mucilage or an organic acid could not be 
ascertained. When the pith is pressed the water exudes in 
small drops. The peculiar translucency of the cells indicates 
a large liquid content. The wood and medulla are very inti- 
mately connected together. The cortex can be removed 
from the stem with ease, leaving the wood and pith closely 
united. It is extremely hard to sever this connection without 
tearing the pith. As will become evident when the histology 
of the stem is presented, a number of the xylem bundles run 
a considerable distance into the pith, and thus bind the 
medulla to the woody cylinder. 
* Examination of the above mentioned material at this writing, December 24, 
1897, sixteen months after collection, shows that all of the plants thus preserved 
are practically unchanged ; a high encomium to pay a preservative liquid which 
stood in an open barrel covered by a lid and a cloth for that length of time. 
Botanists, therefore, who go to the tropics, should carry formalin, in preference to 
alcohol and the ordinary felt driers, because specimens preserved in formaldehyd 
can be dried and mounted for herbarium purposes with but slight loss of color 
upon reiurn to civilization. 
