and Histogeny of Strychnos . 285 
only one cell is usually cut off on the side towards the wood. 
The secondary sieve-tubes formed by the local cambium have 
inclined walls, each of which sometimes bears three sieve-plates. 
The cambial cells are perhaps best studied in a radial section, 
in which they are easily recognized by their abundant proto- 
plasm and conspicuous nuclei. The sieve-tubes appear to lose 
their contents before obliteration, and sometimes show a little 
callus on their plates. 
In .S', mix vomica, as in vS. spinosa , a hypodermal periderm is 
formed, but in the former species there is little phelloderm. 
The fibres of the pericycle become obliterated, while the 
sclerotic cells increase in number, probably by sclerosis of 
some of the outer cells of the bast parenchyma. The latter 
eventually receives considerable additions from the cambium, 
and at the stage now to be considered is about six cells in 
radial thickness. The external groups of soft-bast become 
almost wholly obliterated by the pressure of the newly 
formed cells. 
Development and Structure of the 
Phloem-Islands. 
The formation of the phloem-islands begins late in the second 
year of growth, at least this was certainly the case in those stems 
where the annual rings could be distinguished with certainty. 
At this period the wood is from thirty-five to forty-five cells 
in radial thickness. As regards the position of the phloem- 
islands relative to the primary bundles we have found no 
general law. The position is best determined with reference 
to the medullary phloem-groups which always lie immediately 
inside the primary xylem-strands. Sometimes the first 
formed phloem-islands lie opposite every alternate medullary 
group, and later on a second ring of islands is formed alter- 
nating with the first, so that every medullary group now has a 
phloem-island corresponding to it. But this does not always 
hold good, for sometimes even the first formed islands lie in 
