TRANSFORMATIONS OF THE CELLS. 



WOODY TISSUE OF THE LIBER. The cell walls of this 

 tissue are generally much bent and thickened, but they 

 rarely exhibit either dots or spiral fibre. Such markings, 

 however, are found in the bass-cells of our common Milk- 

 weed (Asclepias), and also in those of the Oleander, a 

 plant of a nearly allied order. By a knowledge of this 

 fact, Professor Schleiden, of the University of Jena, was 

 enabled to determine with precision what plant had pro- 

 duced the fibre in a bit of string that had been tied 

 round a wine vase found in Pompeii, which proved to be 

 our common Milkweed (Asclepias Syriaca), Thus in- 

 delibly are the great truths of Nature inscribed, and in 

 so small a thing as the fibre in a bit of cord, they may be 

 read without mistake, even after the lapse of ages. 



43. DUCTS. Sometimes an uneven thickening of the 

 cell walls occurs, so that they become dotted, and marked 

 in various ways. To all these varieties of tissue the 

 general name of DUCTS may appropriately be given. 

 They are often called Vessels, and the textures which 

 they form, the Vasiform and Vascular tissues. But 

 these terms are very loose and improper, since the vege- 

 table cells are not continuous tubes, like the veins and 

 arteries of animals ; but, on the contrary, they are com- 

 pletely isolated, like other forms of the cell, being always 

 interrupted, at certain intervals, by closed points, and 

 giving passage to the liquids which they receive and 

 transmit, only through their permeable walls. There are 

 ^everal varieties of the duct, which may be distinguished 

 by their different markings. 



44. DOTTED DUCTS have also been termed Pitted or 

 Vasiform Tissue. Sometimes they appear in tubes of 

 considerable length, with their ends overlapping each 

 other, as in fig. 3 ; again, they are marked by imperfect 

 transverse partitions, which plainly show their cellular 

 origin, as in fig. 13. Ducts are the largest tubes in the 

 vegetable fabric, and their wide, open mouths are con- 

 spicuous, even to the naked eye, in a cross section of 

 many kinds of wood, as the Chestnut, Oak, and Mahog- 

 any. Dotted ducts are seldom found, except in wood, 

 and there they greatly abound, often constituting a con- 

 siderable portion of its substance. Their size, as com- 

 pared with the tubes of common woody tissue, may be 

 seen in the transverse section A a at fig. 2, where they 

 are represented by large white spots the minute dots 

 being the divided wood-cells. In the vertical section JB, 

 d indicates Spiral Ducts, b b b Dotted Ducts, c c c 

 wood-cells, and efgh various forms of cells in the bark. 



In this figure, which represents a section of a woody 

 stem of three years' growth, the successive bundles of 

 Ducts and Wooden tubes are well shown. The letters 

 in the transverse section, A, refer to the same parts. 

 Transverse means in a cross direction, or parallel with 

 the plane of the horizon. Longitudinal or vertical means 

 lengthwise, or in a direction at right angles with the 

 plane of the horizon. As these terms are in very com- 

 mon use through the whole study, you will do well to 

 remember them. There are no Duets in the Pines, and 

 in this tribe alone is the wood composed wholly of the 

 proper woody tissue. 



45. SPIRAL DUCTS bear a general resemblance to 

 woody tissue, but they are much longer and more delicate, 

 being, in fact, the longest cells in the vegetable body. 

 These very slender tubes are formed by the confluence of 

 several cells tapering to each extremity, and having 

 their walls strengthened by spiral fibres, which are coiled 

 up on the inside. If you will take a stalk of Geranium or 

 Strawberry, or almost any growing and tender leaf-stalk, 

 and make a slight incision round, without separating the 

 parts, and then gently draw them asunder, you will see 

 the spiral fibre uncoiling itself, like the filaments of a fine 

 web. Generally the spire is composed of a single fibre, 

 as at a in fig. 12 ; often of several, and seldom of two, as 

 at b in the same figure, which represents a spiral fibre 

 from the stalk of a Melon, with the tubes at a overlapping 

 each other. 



46. The situation of spiral ducts is in the MEDUL- 

 LARY SHEATH, or that circle of cells that immediately 

 surrounds the pith and its ramifications, in the stem, the 

 stalk and veins of leaves, and all parts which are modi- 

 fications of leaves. They contain and transmit air, and 

 may be seen in their true place at d, in the vertical 

 section, fig. 2. 



47. ANNULAR DUCTS. Sometimes the fibre is broken 

 into distinct rings, and these are termed ANNULAR DUCTS. 

 They may be seen at fig. 11, where the cellular origin is 

 well shown, by the remains of the transverse partitions. 

 In figures 8, 9 and 10, are represented three cells 

 marked by Dotted, Annular and Spiral Ducts. 



48. SCALARIFORM DUCTS. These abound in Ferns. 

 They are so named because the lines or slits in the old 

 tissue form transverse bars, like the rounds of a ladder. 

 They may be considered as a variety of the Dotted Duct, 

 with the dots or thin places lengthened and more 

 equally distributed. The office of all these last forms of 



In what part fouud ? Observation of ScMeid >n. Ducts. By what caused ? 

 Why a better name than vessels ? Dotted ducts. What sometimes called ? 

 fn what part fouud ? What wood has no ducts ? Spiral Ducts, what resem- 



ble? How formed? The experiment. Where found? Contain and transmit 

 what? Annular Ducts. What do they show? Scalariform Ducts. Why so 

 called? 



