282 Scott and Brebner . — On the Anatomy 
phloem of Strychnos contains sieve-tubes or not. This 
question can be best investigated at a stage slightly later 
than that just described. In transverse sections the hori- 
zontal sieve-plates can be recognized as dotted areas, while 
the longitudinal section shows that these plates are per- 
forated. Normal companion cells accompany the sieve- 
tubes. The latter, however, sometimes contain nuclei, 
which in most sieve-tubes disappear at an earlier stage, and 
this fact may point to the rudimentary character of the 
normal phloem here (see Figs. 2 and 3). 
Later stages of development from the same plant show that 
the cambial divisions go on in those cells which are in im- 
mediate contact with the external phloem. Even when the 
wood has increased to a thickness of sixteen to eighteen 
cells, no considerable addition has taken place on the outside 
of the cambium. The outer layers of the pericycle undergo 
sclerosis, while its prosenchyma first becomes thick-walled, 
and is then obliterated. The structure of the pericycle is a 
character which differs very much among the different species 
of the genus. Other material, probably of the same species, 
but from a different source, showed the formation of a hypo- 
dermal phellogen producing cork centripetally, and abundant 
phelloderm centrifugally. The phelloderm contains many 
sclerotic cells. The normal cambium increases its activity, 
and produces large quantities of bast parenchyma and short- 
celled sclerenchyma. At this stage the stem has no less 
than four distinct consecutive zones of mechanical tissue in 
its cortical region. Towards the exterior are the sclerotic 
cells of the phelloderm, then those of the pericycle, next the 
fibres of the pericycle (soon becoming obliterated), and lastly 
the secondary sclerenchyma of the bast. Cambial increase 
of the medullary phloem groups soon begins, by means of 
divisions in the cells situated towards the protoxylem, and a 
certain amount of obliteration on the inner side of the 
group is the result. 
Petersen 1 found that in a large proportion of the plants 
1 1. c. p. 395. 
