1 88 Le Goc. — Observations on the Centripetal and Centrifugal 
of io cells (Fig. 6), as compared with 1 6 or 18 in a leaf two years old 
belonging to the same plant (Fig. 2). 
Apart from numerical results, the mere look of the sections would 
force one to conclude that secondary growth occurs in the centrifugal 
xylem at the base of the petioles. This view is confirmed by the examina- 
tion of the cells near the cambium in a moderately old petiole over 1 metre 
long ; the cells at the ends of the rows are evidently undergoing a process 
of lignification, for they are thinner than their neighbours, stain less deeply, 
giving, in fact, all the transitional shades specially visible in sections stained 
with gentian violet and orange G. 
It seems therefore proved that the centrifugal xylem at the base 
of the petiole is a secondary growth, at least in its main bulk, and we have 
to disagree with Dr. Scott, who holds that 6 the centrifugal part of the wood 
must be regarded as a primary structure, though in certain cases it may 
receive subsequent addition from a cambial layer ’. 1 This small allowance 
for secondary growth is at least too limited. 
Following the bundle still higher, we have seen the ring of xylem 
breaking up altogether ; the twoxylems are completely separated by a layer 
of thin-walled cells ; the protoxylem is in contact with the centripetal xylem, 
but not, generally speaking, with the centrifugal xylem (Fig. 10). No centri- 
fugal xylem is found in young petioles at this level where the centripetal 
xylem is well developed. It is produced later than the centripetal xylem, 
and appears to be in continuity with the radial rows of meristem cells. 
There is at least no definite line between the large amount of undoubted 
secondary centrifugal xylem at the base of the petiole and the much 
smaller amount found at the higher level, and we are inclined to agree with 
Dr. Carano, who regards the latter as also of secondary origin : ‘ d’origine 
secondaria s’intende bene, ultimi avanzi del legno centrifugo.’ 2 
From these observations I am led to inquire into the relations between 
the centripetal xylem and centrifugal xylem. I shall deal with them only 
in the light of my own observations. 
The gradual bending backwards of the lateral centrifugal xylem as one 
passes away from the base would seem to favour the inversion theory 
(Figs. 4 and 9). There is also a tendency in the phloem to follow this 
motion, though, in fact, it lags a good deal behind. But perhaps this inver- 
sion is more apparent than real. The theory is based upon an assumption 
which needs proof — namely, that it is the same xylem which takes up these 
different positions. 
Not only is proof lacking, but the facts themselves seem to contradict 
the hypothesis. We have seen that the centrifugal xylem at the base of 
the petiole is a secondary growth, at least in its main bulk, or, with the 
1 W. C. Williamson and D. H. Scott (’96). 
2 E. Carano (’OS). 
