. Text-book of botany, morphological and physiological. Botany. FORMS AND SVSTEMS OF TISSUES. 89 of them as a special form of coalescence of cells. They make their appearance in the young phloem in the form of long tubes arranged in rows, with thin walls and trans- verse or oblique septa, on which a net-work of thickening-ridges is soon observed, enclosing thinner areolae. At a later period these latter appear to be actually perforated, while the thickening-ridges between them often swell up enormously. In this con- dition the septum, perforated by a number of pores, is termed a Sieve-plate; i
. Text-book of botany, morphological and physiological. Botany. FORMS AND SVSTEMS OF TISSUES. 89 of them as a special form of coalescence of cells. They make their appearance in the young phloem in the form of long tubes arranged in rows, with thin walls and trans- verse or oblique septa, on which a net-work of thickening-ridges is soon observed, enclosing thinner areolae. At a later period these latter appear to be actually perforated, while the thickening-ridges between them often swell up enormously. In this con- dition the septum, perforated by a number of pores, is termed a Sieve-plate; it is usually broader than the diameter of the tube, which therefore appears dilated at its septa, the sieve-plates, and hence acquires a very characteristic form (Fig. 74). Sieve- plates of simpler structure are also usually formed in the side-walls where two sieve- tubes come into contact. In their early stage the sieve-tubes usually contain a tough albuminous mucilage very little affected by various solvents, which accumulates on both sides of the plate, and fills up the pores. The peculiar configuration of the sieve-plates, and the difficulty of obtaining longitudinal sections of them, render the observation of these characters extremely difficult; this is especially the case with the perforation of the sieve-plates, which can, however, be proved by a method first employed by myself1. It is suffi- cient to saturate - thin longitudinal sections of the phloem with iodine-solution until the contents of the sieve-tubes begin to turn brown, and then to add concentrated sulphuric acid; this dissolves the cell-walls and the substance of the sieve-plates, and nothing is left but the mucilaginous contents coloured a deep brown. The accumulations of protoplasm on each side of the sieve-plate are now seen to be united by slender threads of the same substance (Fig. 74, p), which evidently previously filled the perfora- tions or Sieve-Pores; and their continuity proves that the pores act
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectbotany, bookyear1882
