498 Thoday . — On the Organization of Growth and 
in transverse section two cells are distinguishable by their larger size and 
less dense contents, one lying outside the other near the inner margin of 
the procambial crescent and in the median line. By the time the number 
of cells in the row has reached about four, the innermost has been 
differentiated as a vessel, and two other similar rows, each of two or three 
cells, are distinguishable on either side of the first row a little removed from 
it. Text-fig. 7, a , represents a slightly later stage. The first vessel has been 
differentiated in each of the lateral rows and the third in the median row, 
where the two oldest already show signs of weakening. Laterally, still 
other rows have been initiated. 
Between the rows of vessels the cells cut off by tangential divisions 
0“ ' ' 'o'oi ' ' o' I mm 
C 
Text-fig. 7. a. Xylem and parts of adjoining tissues of a young bundle near the apex of the 
same plant as Text-fig. 6, showing ‘ cambial 7 origin of protoxylem. b. Same bundle on a smaller 
scale : /, fibre initials ; p, differentiating phloem ; pc., procambium ; c, canals associated with the 
bundle, c. Young median trace bundle just above the cotyledons, belonging to one of the leaves 
at the second epicotylar node of a young plant. Here also the protoxylem is ‘ cambial 7 in origin. 
divide further by radial walls, particularly those in immediate proximity to 
the vessels, and thus form round the rows close sheaths of small cells with 
dense contents. Midway between the rows the cells divide less and their 
contents become less dense. In a mature bundle the sheaths are collen- 
chymatous and conspicuously chlorophyllous, 1 without air-spaces. For 
variable distances from the pith they are separated by narrow bands of 
larger cells with air-spaces but less chlorophyll. 
At the stage corresponding to Text-fig. 7, a , the differentiation of 
phloem was already proceeding in the median part of the procambial 
crescent, but the ends of the crescent were still in the procambial condition 
and actively growing (Text-fig. 7, b) '. The crescent extends in this way at 
both ends and the differentiation of phloem follows in both directions from 
1 Indeed, tbe vascular zone is the greenest part of the stem ! 
