Fig. 41.—Monostele in which the Central Pith is Star-shaped, and the Wood breaking up into Separate Groups
p, Pith; W, wood; P, phloem; E, endodermis; C, cortex.
Referring again to [fig. 37] as a starting-point, imagine the pith in the centre to spread in a star-shaped form till the points of the star touched the edges of the ring, and thus to break the wood ring into groups. A stage in this process (which is not yet completed) is shown in [fig. 41], while in [fig. 42] the wood and bast groups are entirely distinct. In the flowering plants the cells of the endodermis are frequently poorly characterized, and the pith cells resemble those of the cortical ground tissue, so that the separate groups of wood and bast (usually known as “vascular bundles”, in distinction from the “steles” of [fig. 40]) appear to lie independently in the ground tissue. These strands, however, must not be confused with steles, they are only fragments of the single apparently broken up stele which runs in the stem.
Fig. 42.—Monostele in which the Pith has invaded all the Tissues as far as the Endodermis, and broken the Wood and Phloem up into Separate Bundles. These are usually called “vascular bundles” in the flowering plants
Fig. 43.—Showing actively growing Zone c (Cambium) in the Vascular Bundles, and joining across the ground tissue between them
The vascular bundle, of all except the Monocotyledons, has a potentiality for continued growth and expansion which places it far above the stele in value for a plant of long life and considerable growth. The cells lying between the wood and the bast, the soft parenchyma cells always accompanying such tissues, retain their vitality and continue to divide with great regularity, and to give rise to a continuous succession of new cells of wood on the one side and bast on the other; see [fig. 33], c, b. In this way the primary, distinct vascular bundles are joined by a ring of wood, see [fig. 43], to which are added further rings every season, till the mass of wood becomes a strong solid shaft. This ever-recurring activity of the cambium gives rise to what are known as “annual rings” in stems, see [fig. 44], in which the wood shows both primary distinct groups in the centre, and the rings of growth of later years.
Cambium with this power of long-continued activity is found in nearly all the higher plants of to-day (except the Monocotyledons), but in the fern and lycopod groups it is in abeyance. Certain cases from nearly every family of the Pteridophytes are known, where some slight development of cambium with its secondary thickening takes place, but in the groups below the Gymnosperms cambium has almost no part to play. On the other hand, so far back as the Carboniferous period, the masses of wood in the Pteridophyte trees were formed by cambium in just the same way as they are now in the higher forms. Its presence was almost universal at that time in the lower groups where to-day there are hardly any traces of it to be found.
