256 
ERNST F. ARTSCHWAGER 
Ontogeny 
Almost immediately below the growing point the procambium becomes 
distinct, forming a concentric band of tissue between cortex and pith. 
At certain points in this procambium the primordia of vascular strands 
appear recognizable as such by the small size of the elements and by their 
granular content. The smallest primordia contain only elements of the 
phloem. Slightly older groups also contain elements of the protoxylem. 
The first formed phloem cells are thin-walled, very narrow, and take the 
typical haematoxylin stain. Sieve tubes become distinct in maturer 
sections only. The protoxylem elements are also narrow and of the loose 
spiral type (PI. XVI, E). The groups of phloem and xylem are at first 
separated by undifferentiated procambium. With enlargement of the 
group, a cambium develops which later initiates secondary growth. 
The procambium surrounding the strands of vascular tissue undergoes 
active division, causing the primary bundles to become separated and 
initiating the formation of new bundles in the widening spaces. These 
new bundles naturally do not extend so far into the pith (PI. XVII, D), 
and, like the primary bundles, they increase in size through the activity of 
a cambium. 
Gradually the procambium cells between the vascular groups cease 
dividing. All the elements mature except a single layer which remains 
meristematic, and which, as an interfascicular cambium, unites the separate 
bundles into a vascular ring. This cambium layer appears at first only 
between the larger groups, while later the smaller and more distant groups 
may also become united to form a part of the primary cylinder. 
During the enlargement of the primary bundles an extrafascicular cam- 
bium appears (PI. XVII, A, C) in the still undifferentiated procambium 
on the outer face of the primary bundles. This layer of meristematic 
cells is not formed simultaneously in a given circle, and as a consequence, 
Fig. 2. Chenopodium album: Drawings illustrating the mode of origin of the intraxylary 
phloem. A, transverse section of stem showing cambium cells dividing in different planes, 
forming eight cells which become the phloem of the vascular ring. B, transverse section of 
stem showing the appearance of a new cambium in the parenchyma adjacent to the newly 
formed ohloem groups. 
