Stelar Theory

the theory describing the structure of and the interrelationships between various types of steles in higher plants. The greater part of the stele is composed of vascular tissues—xylem (wood) and phloem (bast)—which are arranged differently in different types of steles. The vascular tissues are usually surrounded by pericycle, which consists of mechanical and parenchymatous cells. Surrounding the stele is the primary cortex.

Study of the stele was initiated in 1886 by the French botanists P. Van Tieghem and H. Douliot, who developed the first classification of steles. Stelar theory was further developed by the British botanist G. Brebner (1902) and the American botanist E. Jeffrey (1903, 1917), who significantly developed the classification of steles. Subsequently, the German scientist W. Zimmermann and the Soviet scientists K. I. Meier and A. L. Ta-khtadzhian noted changes in the stele in the ontogeny and phytogeny of plants.

The simplest type of stele—the protostele—is characteristic of the oldest higher plants—psilophytes (for example, Rhynia). The protostele has a central strand, inside which is xylem surrounded by phloem; the phloem is not clearly delimited from the primary cortex. According to the English botanist F. Bower, the development of the stele structure in plant evolution paralleled the development of a more efficient ratio between the volume and surface area of vascular tissues. The more efficient ratios were the result of changes in the contours of the stele and led to segmentation of the stele into separate strands. An important role in the evolution of the stele was played by the process of medullation—the formation of pith—and by the appearance of live parenchymatous cells in the vascular tissues.

The development of the stelar structure of plants was also accompanied by differentiation of the procambium into protoxy-lem, metaxylem, protophloem, and metaphloem and by changes in the character of deposition of xylem from mesarch to exarch and endarch. With mesarch deposition, the first elements of the xylem are differentiated in the center of the procambial strand, and all successive elements are differentiated in radial directions. Exarch xylem is characterized by peripheral deposition of the first elements and centripetal development of successive elements. In endarch xylem, the first elements are formed from the internal part of the procambium, and successive elements develop centrifugally. Other significant processes included the segmentation of the stele into stem and leaf bundles that enter the stem from the leaf, the strengthening of the bonds between stem and leaf bundles, and the formation of the endodermis as a barrier to prevent loss of moisture and retain the products of assimilation in the stele. There has been a general increase in the size of the stele in the evolution of terrestrial plants.

Different types of structural organization of the stele arose as a result of various trends in plant evolution. Thus, changes in the contours of the xylem caused a transformation of the protostele into an actinostele or a plectostele. The actinostele has exarch xylem that exhibits a lobular cross section; it is characteristic of psi-lophytes (Asteroxylon) and, among extant plants, Psilotum. In a plectostele, which is typical of Lycopodium, the exarch xylem is divided into ribbonlike strands.

A system of vascular tissues that form the cylinder surrounding the parenchymatous pith is characteristic of a siphonostele. Ferns may exhibit any one of three types of siphonostele: ectophloic siphonostele, amphiphloic siphonostele (solenostele), and dictyo-stele. The ectophloic siphonostele apparently developed from the actinostele with the drawing in of spurs of xylem, concrescence of areas of phloem into a continuous ring, and development of the parenchymatous pith, whose cells originated from tracheids that lost the ability to conduct water and were divided by transverse septa. The xylem is surrounded by phloem, pericycle, and endodermis, as in Helminthostachys. A solenostele has external and internal phloem, pericycle, and endodermis, as in Marsilea. Meier’s research on the development of the vascular system in ferns has demonstrated the possibility of formation of the solenostele from the ectophloic siphonostele.

The dictyostele originated as a result of marked dissection of the amphiphloic siphonostele caused by the appearance of numerous leaf gaps filled with parenchyma. Its shape—that of a reticulated cylinder—is caused by its vascular tissues, which form tangled strands called meristeles. A cross section of a stem shows the meristeles arranged in a ring around the pith. They are arranged like concentric amphicribral bundles, in which phloem is arranged around the xylem and is in turn surrounded by the pericycle and endodermis.

Growth changes in the stele of ferns (Marattia, Pteridium, and Matonid) are seen in the formation of a second and, subsequently, a third stele in concentric layers inside the first. According to Zimmermann, the transition from protostele to polystele led to the formation of a eustele, in which each protostele has been transformed into a collateral bundle. Some botanists believe that the eustele could have been formed from the ectophloic siphonostele, whose segmentation into separate bundles was caused by the formation of medullary rays.

In equisetums, the eustele is represented by closed collateral bundles arranged around a central air cavity and joined at nodes. A lacuna, or water-translocation cavity, is formed in a bundle in places where the xylem is destroyed early. This type of eustele is called a horsetail stele. In different species of equisetums, the stele is characterized by various arrangements of the endodermis. Research on the early stages of development of the vascular systems of equisetums supports the belief that the horsetail stele developed from an actinostele or siphonostele that separated into individual bundles.

The eusteles of seed plants are characterized by more developed leaf bundles and the close contact between stem and leaf bundles. The eustele in dicotyledons is represented by a system of open collateral or bicollateral bundles with endarch primary xylem. The bundles are dissected by parenchymatous medullary rays that intersect the stele radially. In many herbaceous plants, the medullary rays are wide; in woody plants, they are narrow, sometimes in single file, and lack an endodermis and pericycle.

In monocotyledons, the highly developed vascular bundles in leaves enter the stem and are dispersed throughout the cross section, and the stem bundles are reduced. As a result, the eustele has been transformed into an atactostele, which has lost the capacity for secondary growth. The vascular bundles in an atactostele are collateral or concentric amphivasal bundles, in which xylem surrounds the phloem.

Stelar theory demonstrates the essential differences between the principal divisions of higher plants according to the structure of their vascular systems. It has proved of great significance in the study of plant anatomy and phytogeny.