1. What structural characteristics make collenchymas particularly adapted for the support of growing regions? Collenchyma walls begin to thicken early during the development of the shoot and, because the cells are capable of increasing simultaneously the surface and thickness of their walls, they can develop and maintain thick walls while the organ is still elongating. In addition, because the wall thickenings are plastic and capable of extension, they do not hinder the elongation of stem and leaf. In celery petioles the collenchyma cells lengthened by a factor of about 30 while the walls strongly increased simultaneously in thickness and surface area (Frey-Wyssling and Mühlethaler, 1965).
2. Describe the development of collenchymas tissue.
The walls of collenchymas cells begins to thicken during the development of the shoot. In a more advanced state of development collenchyma continues to be a supporting tissue in plant parts (many leaves, herbaceous stems) that do not develop much sclerenchyma. With regard to the supporting role of collenchyma, it is of interest that in developing plant parts subjected to mechanical stresses (by exposure to wind, attachment of weights to inclined shoots), the wall thickening in collenchyma begins earlier than in plants not subjected to such stresses (Venning, 1949; Razdorskii, 1955; Walker, 1960). In addition stressed shoots may exhibit a signifi cantly greater proportion of collenchymas (Patterson, 1992). Such stresses do not affect the type of collenchyma formed.
3. What is known about the ultrastructure of collenchymas cell walls? The walls of collenchyma cells are thick and glistening in fresh sections (Fig. 7.11), and often the thickening is unevenly distributed. They contain, in addition to cellulose, large amounts of pectins an hemicelluloses and no lignin (Roelofsen, 1959; Jarvis and Apperley, 1990). In some...