Why is celery crunchy?
When we eat a piece of celery, we are eating the stem, or petiole of the plant. (see Fig. 1). The crunchiness of the stalk is due to the types of cells that are contained in celery tissue.
Fig. 1 The crunchy part of the celery that we eat is the stem of the plant. Image from Thomé. Flora von Deutschland, Österreich und der Schweiz. 1885.
Besides xylem (water-conducting) and phloem (food-conducting) tissues, which together are called vascular bundles, celery contains collenchyma tissue, which provides support for the plant. Collenchyma tissue is made up of elongated living cells filled with water, and the pressure of the water against the cell walls creates a stiffness that gives celery its crunch.
Collenchyma is found in plant stems, leaves and flowers. On a stem of celery the collenchyma tissue is located on the ribs along the outside of the stalk and on the inside (Fig. 2).
Fig. 2 Close up of celery showing the darker green collenchyma tissue along the outer edges of the stalk and the vascular bundles. Photo by Fir0002/Flagstaffotos, courtesy of Wikimedia Commons and the GNU Free Documentation License.
Plant anatomist Katherine Esau studied the structure of tissues in celery. Her photos below show a cross section of a celery stalk. The collenchyma tissue forms the pointed ribs along the bottom, and the vascular bundles above it look like upside-down acorns.
Left: Cross section of celery petiole, x11. Right: Cross section of celery petiole. Photos by Katherine Esau.
Why does a pear feel gritty when you chew it?
Pear fruit contains schlerenchyma tissue composed of sclereid cells. The origin of the cells' name derives from the Greek word sklerosis, meaning "hardening." The cells have a secondary cell wall filled with lignin, an organic substance that provides support and hardens the tissue. Sclereids are found throughout many plants and are what make a nut, shell, or apple seed hard. They are also located throughout the fleshy part of the pear fruit and are sometimes called "stone cells."
Cross section of a pear, x6.5 times. Photo by Katherine Esau.