Origins and nature of vessels in monocotyledons. 13. Scanning electron microscopy studies of xylem in large grasses

Scanning electron microscopy of thick hand sections of the giant thick-stemmed grasses Arundo donax and Phyllostachys aurea was used to see how the primary xylem of a monocot with pronounced basal meristem activity in stems differs from xylem in monocots that show gradual elongation in stems. Several intact protoxylem vessels with annular thickenings can be simultaneously present in a single bundle, indicating that redundancy is a possible mechanism for countering loss of integrity resulting from elongation stress. Protoxylem vessels can be large in diameter, even larger than adjacent metaxylem vessels. There is little or no network of cellulosic fibrils in the primary walls (including end walls before dissolution) of protoxylem vessels, suggesting that primary walls are essentially gelatinous and therefore may elongate more readily than other vessel elements studied to date. Annular vessels are much more common than helical ones. Other notable structural features reported are smoothness of the inside of primary walls in protoxylem vessels, absence of borders on annuli or gyres of protoxylem vessels (but presence of borders on rims of perforation plates), and grooves interconnecting one to 10 pit apertures (coalescent pit apertures) on the inside surfaces of metaxylem vessels. Use of thick sections for study with scanning electron microscopy permits imaging that is ideal for understanding three-dimensional aspects of xylem anatomy and is recommended over the study of thin sections.