1913] CHRYSLER—PHLOEM OF ABIETINEAE 49 
series of these cells are more elongated than the ones formed later, 
but all possess the same histological features and undergo the same 
fate. Similar in form and contents are the cells which span the 
space between two vertically contiguous rays, and which by fission 
and shortening give rise to a row of erect cells on each of the 
two rays. 
Since the cells of the radial rows occur principally in the young 
plant, and have the same contents as sieve tubes, with the exception 
of the nucleus, it is a tempting theory to consider them the fore- 
runners of sieve tubes, the primary sieve cells which undergo evolu- 
tion in two directions: (1) lengthening out and losing the nucleus so 
as to produce sieve tubes, and (2) shortening as they become applied 
to medullary rays and become converted into erect cells or as they 
give rise to rays independently. But such a theory must stand on 
the evidence of a comparative study of phloem of vascular plants, 
and the evidence is not yet at hand. 
Summary 
1. The “erect cells” occurring on the margins of medullary rays 
in the phloem of most genera of Abietineae do not exist i in the young 
tay, which consists only of ordinary parenchyma. 
2. In young roots of Pinus the phloem shows certain cells which 
are essentially short sieve tubes possessing nuclei, occurring in 
groups in the radial plane. On the xylem side these may merge 
into one or more narrow rays consisting of tracheids, owing to a 
diminution and localization of cambial activity. 
3- When such a radial group occurs in vertical contact with a 
medullary ray, cambial activity is sooner or later localized at the 
edge of the ray, resulting in the production of a border to the ray, 
such border consisting of sieve cells, which are the erect cells found 
in mature phloem. 
4. Variations of this mode of origin of erect cells occur, such as 
the cutting of a cell from the end of a sieve tube when it meets the 
edge of a ray. 
5. In young roots and stems sactiel cells may make their 
appearance in the phloem earlier than in the xylem, while in Abies 
