Taken down the barrel of a monocular microscope, with an iPhone, by a year 11 student! She was very pleased with this photo, and I was very pleased that she thought celery interesting enough to photograph. The darker stained spot is a vascular bundle, comprised of xylem and phloem vessels separated by vascular cambium (a section of cells whose job is to produce more phloem and xylem cells). The two lighter spots toward the top left hand side of the picture are tears in the section.
(TS of celery stem;stained with methylene blue)
Some basic plant knowledge:
Phloem and xylem vessels are a little bit like blood vessels in humans because they are the transport system inside a plant. Xylem vessels carry water from the roots of a plant up to the leaves where that water evaporates through the pores on leaves (stomata). The process of water moving through a plant is called transpiration and it is driven by the evaporation at the leaf. Water molecules cohere to one another and adhere to the walls of the xylem vessels, so as water is evaporated at the stomata, the thin stream of water molecules is drawn up through the xylem. Xylem vessels are composed of dead cells that link together to form long tubules stretching through the plant. They are heavily lignified, or woody and water can only go up, not down. Dissolved in that water are also some minerals from the soil.
Phloem vessels transport organic nutrients (sugars) around the plant. These cells are living tubules that have lost their cell contents to allow the phloem sap to pass through. Companion cells can be found next to phloem cells to feed the cell and to load it with sugar. Phloem sap can flow upwards and downwards using a mechanism known as ‘source to sink’. The source of the sugar is any cell of the plant that is photosynthesising. The sink is any part that needs sugar for respiration. The sugar moves from where it is made, at the source, to where it is required, at the sink.