The next type of stele is quite similar to the protostele, but with the addition of a few large unspecialized cells in the middle of the wood (p, [fig. 37]); these are the commencement of the hollowing process which goes on in the wood, resulting later in the formation of a considerable pith, as is seen in [fig. 38], where the wood is now a hollow cylinder, as the phloem has been from the first. When this is the case, a second sheath or endodermis generally develops on the inner side of the wood, outside the pith, and cuts the vascular tissues off from the inner parenchyma. A further step is the development of an inner cylinder of bast so that the vascular ring is completely double, with endodermis on both sides of the cylinder, as is seen in [fig. 39].
Fig. 39.—A Cylindrical Stele, with e, inner endodermis, and ph, inner phloem; W, wood; P, outer phloem; E, outer endodermis. L, part of the stele going out to supply a large leaf, thus breaking what would otherwise appear as a closed ring stele
In all these cases there is but one strand or cylinder, of vascular tissue in the stem, but one stele, and this type of anatomy is known as the monostelic or single-steled type.
Fig. 40.—A Ring Stele apparently broken up into a Number of Protosteles by many Leaf Gaps
When from the double cylinder just described a strand of tissue goes off to supply a large leaf, a considerable part of the stele goes out and breaks the ring. This is shown in [fig. 39], where L is the part of the stele going to a leaf, and the rest the broken central cylinder. When the stem is short, and leaves grow thickly so that bundles are constantly going out from the main cylinder, this gets permanently broken, and its appearance when cut across at any given point is that of a group of several steles arranged in a ring, each separate stele being like the simple protostele in its structure. See [fig. 40]. This type of stem has long been known as polystelic (i.e. many-steled), and it is still a convenient term to describe it by. There has been much theoretical discussion about the true meaning of such a “polystelic” stem, which cannot be entered into here; it may be noted, however, that the various strands of the broken ring join up and form a meshwork when we consider the stem as a whole, it is only in a single section that they appear as quite independent protosteles. Nevertheless, as we generally consider the anatomy of stems in terms of single sections, and as the descriptive word “polystelic” is a very convenient and widely understood term, it will be used throughout the book when speaking of this type of stem anatomy.
Such a type as this, shown in [fig. 40], is already complex, but it often happens that the steles branch and divide still further, until there is a highly complicated and sometimes bewildering system of vascular strands running through the ground tissue in many directions, but cut off from it by their protective endodermal sheaths. Such complex systems are to be found both in living and fossil plants, more especially in many of the larger ferns (see [fig. 88]).
Higher plants in general, however, and in particular flowering plants, do not have a polystelic vascular arrangement, but a specialized type of monostele.
