PHOTO STUDY 15-1 Salix (willow): l.s. young stem, below apex

This study is taken from the fourth or fifth node below the stem apex, where leaf base and axillary bud are shown, with vascular tissue entering both leaf and bud from the stem. Observe the vascular bundle (leaf trace) that departs from the stele and gradually ascends through the cortex, finally entering the leaf base. See the relative positions of xylem and phloem in the leaf trace, and recognize the parenchyma tissue of pith and cortex. Immediately above the leaf trace, identify two procambium strands that will enter the axillary bud. These are branch traces. Note that there is no continuation of the stele above the departing leaf trace. At a level in the stem slightly above the bud, you can see stelar tissues again, quite in line with the bundle that departed to the leaf. This apparent interruption in the continuity of the stele is a leaf-and branch gap.

PHOTO STUDY 15-2 Salix (willow): t.s. stem, Series 1 below a node

Transverse sections of the stem to be studied now are in serial order. From this section, follow upward in the series, watching the petiole (leaf primordium) as it departs from the stem. Then you see the blending of leaf base with the stem. The base of the axillary bud between stem and leaf base makes its appearance with two procambial strands. Finally, toward the tip of the bud it fuses becoming the vascular system of the bud. At the level of this photo, three traces can be identified in the stele. Is the boundary between pith and xylem an even circle, or do some areas of the xylem dip more deeply into the pith than others? Can you account for this? Identify cortex and epidermis. The very deeply stained contents of a great many cells of the section are probably tannins.

PHOTO STUDY 15-3 Salix (willow): Series 2 below a node

Transverse section of the stem at a level where the median leaf trace has just left the stele, and where the two laterals are just beginning to leave. Is this level above, or below, the position of the section represented in your preceding study? Do all traces depart from the stele at the same level, or not? Note the vacancy in the stele left by the departure of the median trace. What is this vacancy called? Is it an empty space, or is it filled with something? What did it look like in the longitudinal section? Note that the pith appears to indent the xylem where the two lateral traces are starting to depart.

PHOTO STUDY 15-4 Salix (willow): nearing a node

Transverse section of the stem at a still higher level. Here, all three traces are out from the stele, leaving behind their respective gaps. Did the three traces depart side-by-side, or were they separated from one another by intervening vascular tissue of the stele? What has happened to the gap? See how the gap opposite the median trace differs from the others. Note that vascular tissue is departing from the sides of the gap, and it appears to be following the trace out into the cortex, though not attached to the trace itself. These are the two branch traces that supply the axillary bud.

PHOTO STUDY 15-5 Salix (willow): at a node

This transverse section is still higher in the stem. The three traces are now in the leaf base, the two lateral gaps being closed by development of vascular tissue, and the two branch traces are coming together to form an incomplete ring (not visible yet here). This, of course, will be the cylindrical stele of the branch stem. If your longitudinal photo showed a very short leaf gap, what would you conclude as to the corresponding trace: was it a median, or a lateral? Does the xylem that fills a leaf gap differ at all from xylem tissue on either side of it?

PHOTO STUDY 15-6 Salix (willow): leaf departing from stem

Note the leaf base is making its appearance, beginning to take on a separate entity, complete with its 3 vascular traces. They are now leaf bundles. Note also the bud is forming and a complete cylinder of vascular tissue will follow.

PHOTO STUDY 15-7 Salix (willow): leaf departed from stem

Transverse section of the stem at a level where the leaf base has separated completely from it, and the axillary bud is now above the plane of section.