Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




A study of tumbleweeds in North Dakota, conducted from June, 1964 to October, 1968, was concentrated on two common species: Psoralea arqophvlla Pursh (silver leaf psoralea) and Kochia scoparia (L.) Schrader (kochia). The objectives were to determine the internal or external factors regulating stem abscission, and to describe the niche of each species through the ecological life history approach. Anatomical and histochemical studies were conducted on the basal portions of stems at various stages of plant maturity. Ecological life history aspects of silver leaf psoralea were studied both in the field and under controlled environmental conditions, while those of kochia were studied in the field only.

Silver leaf psoralea, a leguminous plant, is the most common of the native perennial tumbleweeds of North Dakota, and is a major forb in moderately grazed native prairies dominated by the later-maturing grasses. It was of less importance in heavily mulched native prairies containing a greater proportion of early-maturing grasses, and was absent or uncommon in overgrazed prairies. In native ungrazed prairies, silver leaf psoralea increases under moderate-grazing and decreases under over-grazing. Adaptive characteristics which enable this species to thrive in certain native grasslands, particularly those moderately grazed, include the following: (l) basal lateral buds, which may initiate new shoots after the apex of the primary shoot has been destroyed; (2) deep root system and large tap root for storage of food reserves; (3) dormancy of roots during periods of physiological stress (late summer and drought periods); (4) partially unpalatable foliage, which is expanded over the tops of the shorter grasses, thus enabling the plant to avoid shading; and (5) initiation of extensive lateral roots through which vegetative reproduction occurs.

Abscission in Psoralea argophylla is a chemical process. In a single plant it proceeded through one or more basal intercalary meri- stems located at or near soil level. During senescence a separation layer differentiated within each of these meristems. Within the compound middle lamella of cells constituting the separation layer, insoluble pectic substances were converted into soluble forms. Separation of cells occurred only after removal of the pectic compounds, meristematic activity, and attenuation of the cell walls. Following abscission a protective layer is formed some distance underground at the stem-root junction. Although the plant may be blown for considerable distances from its root, it is not normally an effective tumbleweed because of the absence or paucity of disseminules.

Kochia scoparia, a member of the Chenopodiaceae, is the most ubiquitous of the annual tumbleweeds of North Dakota, and is a serious weed in certain areas. Aside from the tumbleweed habit, adaptive features which contribute to its success in disturbed habitats include: Cl) the rapid growth rate and large size; (2) tolerance to a wide variety of edaphic conditions, including drought- and salt-affected soils; (3) the extensive root system; and (4) tolerance to many weed control methods.

In contrast to silver leaf psoralea, abscission in kochia is a mechanical process. During senescence the wpody plant axis becomes dehydrated, and soil-inhabiting fungi (Rhizoctonia sp.) usually invade the structurally weak abscission zone tissues, causing incipient decay. The tissues become brash, however, primarily as a result of the former process. When the bushy plants are bent by a wind of 25-30 mph, stresses of considerable magnitude are exerted upon the abscission zone at the base of the stem. The initiation of abscission depends largely upon the forces exerted by the wind and extent of desiccation of the plant axis; however, the magnitude of the force may be reduced approximately 50% in plants which have undergone decay for several weeks. The abscised plant may be blown intact for several miles scattering thousands of its highly viable seeds.