Blood pressure, wind, plant tissues and humidity

Plants stand up to wind with three simple tissues. Parenchyma has living cells with thin cellulose walls and large vacuoles that hold water, starch and minerals; it stores food, helps air spaces, and the cells that contain chloroplasts photosynthesise. Not every parenchyma cell is green. Collenchyma has elongated living cells whose walls are unevenly thickened with cellulose and pectin; it gives strength with flexibility in growing parts and lies under the epidermis of young stems and in leaf veins. Sclerenchyma has very thick walls containing lignin, is rigid and impermeable to water, and is mostly dead when mature; it supplies the main strength. A flexible stem bends in the wind and springs back, while a rigid tissue holds the shape. A celery stalk shows this: the tough strings you peel off are strengthening tissue.