MORPHOLOGY OF HUMULUS LUPULUS. II. SECONDARY GROWTH IN THE ROOT AND SEEDLING VASCULARIZATION

MILLER, ROBERT H. (U. Nevada, Reno.) Morphology of Humulus luppulus. 11. Secondary growth in the root and seedling vascularization. Amer. Jour. Bot. 46(4): 269-277. Illus. 1959.In the primary state the roots of Humulus lupulus L. have a diarch xylem plate with 2 strands of primary phloem lying on either side of the primary xylem. Secondary histogenesis is described for the primary root. Fibrous and fleshy storage roots are developed by the hop plant and their respective developmental and anatomical structures are described. Lateral roots are initiated in the pericycle opposite the protoxylem poles. The architecture of these secondary roots is similar to that of the primary root. The seedling develops a fleshy storage organ through secondary growth of the primary root and the hypocotyl. The hypocotyl eventually resembles a fleshy taproot throughout most of its extent. The vascular cambium differentiates large amounts of parenchymatous tissues. A relatively smaller amount of tracheary tissue is formed. The secondary phloem comprises a high percentage of phloem parenchyma and ray cells containing numerous large starch grains, and constitutes the larger portion of the fleshy storage root. Numerous thickwalled lignified fibers occur throughout the secondary vascular tissues. Resin and tannin cells are abundantly distributed. A phellogen is differentiated from the pericycle and develops a persistent periderm on the outer surface of the fleshy storage organ. A relatively short transition region occurs in the upper part of the hypocotyl. The transition takes place from a radially alternate arrangement of the vascular tissues in the root to a collateral arrangement in the cotyledons. THE ONTOGENETIC DEVELOPMENT of the primary tissues in the root of Humulus lupdulus L. was described in detail in a previous paper (Miller, 1958). In the present paper the development of the secondary tissues of the primary root, lateral root formation, and seedling vascularization will be described. As was pointed out in the earlier paper, the literature on the developmental anatomy of hops is limited. Aside from the earlier investigations of Lermer and Holzner (1896) on hop anatomy in general and the more recent and unpublished work of Runner (1950) on hop roots, there have been no developmental studies of the vegetative organs of this economically important plant. MATERIALS AND METHODS. The materials and methods were those presented in the earlier paper. However, mention should again be made of the clearing technique which facilitated the rapid determination of the arrangement of the vascular tissues in the "transition region." Entire young seedlings ('Early Fuggles' and 'Late Clusters' varieties) were aspirated in a 5.25% solution of sodium hypochlorite and then again aspirated in water. They were dehydrated in an ethyl alcohol series, to which safranin alone was added at the 70%7 level, and finally stored in xylene. SECONDARY HISTOGENESIS.-As described in the previous paper, the vascular cambium originates from parenchymatous cells situated between the diarch primary xylem and primary phloem. The first cells of the cambial region are located as isolated strips on each side of the primary xylem plate, 1 Received for publication Novemlber 10, 1958. outwardly bordered by the primary phloem. As secondary xylem is cut off toward the inside of the cambium and secondary phloem toward the outside, the cambium takes on the appearance of 2 "arcs," more or less bulging toward the outside of the vascular cylinder. With further development the cambial region assumes an undulated appearance in its encirclement of the vascular cylinder. With still further development, the cambium becomes a more or less cylindrical layer of tissue around the circumference of the differentiating secondary xylem, except for interruptions at the protoxylem poles. In the meantime, the pericycle immediately abutting the protoxylem poles undergoes periclinal divisions resulting in the differentiation of cambial cells in this region also. With the formation of these additional cells, the "gaps" in the vascular cambium are filled and the cambium completely encircles the xylem. The vascular camibium of the root of Huimulus lZp'iuus is composed of both ray and fusiform initials. Those pericyclic cells abutting the protoxylem give rise to multiseriate ray initials. The developing rays eventually radiate from the protoxylem poles outwardly through the secondary vascular tissues. These are the first multiseriate rays in the secondary xylem. Ray initials are also produced by the cambium between the xylem poles; however, these are at first uniseriate in nature. Runner (1950) reported that the vascular cambium was observed to be of an "anomolous type" with the cambium "-multiseriate" between the vascular rays. "Sieve tubes and companion cells are differentiated in the center of this region., while