Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
144 DESIGN IN NATURE<br />
wall of the daughter-cell has absorbed water and burst ;<br />
a rounded contour (after Sachs).<br />
PLATE LX (continued)<br />
the protoplasmic body having forced itself through the fissure and assumed<br />
Fig. 2.— a. Transverse section through a young internode of the stem of Tradescantia alUfiora. a, Vascular bundles; h,<br />
large cells forming the parenchyma of the fundamental tissue (after De Bary).<br />
B. Transverse section of the flower-scape of Allium Schcenop-asum , x about 30. a, Epidermis ; 6, chlorophyll cells; c, colourless<br />
parenchyma of the cortex ; d, parenchyma of pith ; e e', vascular bundles ; /, ring of sclerenchyma (after Sachs).<br />
Fig. 3._A. Pinus sylvestris. Radical longitudinal section through the wood of a vigorous branch, a, Cambial wood-cell ; 6,<br />
older wood-cells ; c, c', c'", bordered pits of the wood-cells in order of age ; d, d, large pits where the cells of the medullary rays are<br />
in contact with the wood-cells. , , i ,, j<br />
B. Tangential longitudinal section through the wood of Ailanthus glandulosa. a, Vessels ; 6, medullary rays cut across; c, wood<br />
^ ''^a Ipirogyra longata. Several cells of two filaments about to conjugate (a, h). Spiral chlorophyll bands (c) with starch grains<br />
[d), circularly arranged, and small drops of oil. The nucleus (e) of each cell is surrounded by protoplasm, from which threads go to<br />
the cell wall.<br />
D. Cells engaged in conjugation, a, Protoplasm of one cell passing into another cell. At 6,<br />
•<br />
x i<br />
i.<br />
two protoplasmic masses have<br />
already united. At c, the young zygotes are provided with a wall (after Sachs).<br />
While the German histologists maintain that everything proceeds from the cell, they differ as to their definition<br />
of a cell.<br />
Max Schultze avers that a cell wall or envelope is not necessary to the existence of the cell, and Briicke<br />
declares that a nucleus does not necessarily form a part of it. If, however, we ehminate the cell wall and the<br />
nucleus, all that remains of the cell is the matter contained between the two, namely, the cell contents or proto-<br />
plasm, and this is now regarded by Professor Huxley and others as the germinal matter, or physical basis of life.<br />
The history of the cell is instructive.<br />
Schleiden and Schwann (1839), the original promoters of the cell theory, were of opinion that cells were produced<br />
from an amorphous mass (cyto-blastema), contained either within or between pre-existent cells. In this mass<br />
round, structureless, or minutely granular corpuscles appear.<br />
Around these corpuscles, which constitute the nucleus, a cell wall is formed by molecular disposition, and between<br />
the cell wall and its nucleus fluid is stored up. By this process a nucleated cell is produced. From a mass of such<br />
cells, according to Schleiden and Schwann, all the textures may be developed. In their opinion " the cause of<br />
nutrition and growth resides, not in the organism as a whole, but in the separate elementary parts—the cells."<br />
Professor John Goodsir (1845), to whose writings I refer further on, thus expresses himself : " As the entire<br />
organism is formed at first, not by simultaneous formation of its parts, but by the successive development of these<br />
from one centre (the germinal spot of the ovum), so the various parts arise each from its own centre, this being<br />
the original source of all the centres with which the part is ultimately suppUed. From this it follows, not only<br />
that the entire organism, as has been stated by the authors of the cellular theory, consists of simple or developed<br />
cells, each having a pecuhar independent vitality, but that there is in addition a division of the whole into depart-<br />
ments each containing a certain number of simple or developed cells, all of which hold certain relations to one central<br />
or capital cell, around which they are grouped. It would appear that from this central cell all the other cells of<br />
its department derive their origin. It is the mother of all those within its own territory."<br />
Goodsir attached much importance to the nuclei of the cells : these formed his " centres of nutrition." In<br />
this matter he proved himself an advanced pioneer. The nucleus is now known to be the seat of the most extra-<br />
ordinary and important changes in reproduction :<br />
it is also beUeved to be the centre of inheritance. " The<br />
identification of the cell-nucleus as the vehicle of inheritance, made independently and almost simultaneously in<br />
1884-85 by Oscar Hertwig, Strasburger, KoUiker, and Weismann, must be recognised as the first definite advance<br />
towards the internal problems of inheritance through the cell-theory."<br />
Similar in many respects to Goodsir's theory of " centres of nutrition " is Dr. Beale's theory of germinal matter.<br />
Professor Huxley (1853) referred the origin of cells to vacuoles occurring in a homogeneous plasma. These<br />
vacuoles contain the cell wall, nucleus, and contents. The cell wall he termed the periplast, the nucleus the<br />
endoplast. He attaches no importance to the latter, in which respect he entirely differs from Goodsir. He says,<br />
in fact, that not unfrequently the nucleus has disappeared before the really essential changes in the cell begin.<br />
Huxley attaches especial importance to the periplast or cell wall and the matter contained within it. In these,<br />
according to him, all the really essential, vital, physical, and chemical changes occur. It would be easy to write<br />
volumes on the cell theory, but enough has been said to show that authors are not agreed either as to what<br />
constitutes a cell, or as to the more important parts of it. Some think that the essential part is the cell wall or<br />
envelope, others the nucleus, others the substance contained between the nucleus and envelope : some ao-ain are<br />
of opinion that neither cell wall nor nucleus is necessary, and that the contained matter or protoplasm is the only<br />
essential element.