Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/268

234 CONTRIBUTIONS TO, in the leaf, stem, and their metamorphosed parts. The largest which I have seen measured 0·0022 Paris inch in diameter (in Fritillaria pyrenaica) ; the smallest, in the embryonal extremity of the pollen-tube of Linum pallescens, from 0·00009 to 0·0001 Paris inch. In the albumen of Abies excelsa I found the average of several admeasurements of examples, which appeared of equal size, to be 0·00034-0·00059-0·00079. In the young leaves of Crassula portulaca, 0·0003; and in the albumen of Pimelea drupacea, 0·00095-0·001055. Little importance, however, can, on the whole, be attached to these admeasurements, since they increase and diminish, and we cannot determine in what period of its existence the cytoblast may be at the time.

Its internal structure is in general granulous, without, however, the granules, of which it consists, being very clearly distinct from each other. Its consistence is very variable, from such a degree of softness as that it almost dissolves in water, to a firmness which bears a considerable pressure of the compressorium without alteration of form. The more recent its formation, the softer it is; and this also applies to cases in which its existence is merely transitory. It is denser and more sharply defined when it endures throughout the whole vital process of the plant as a permanent tissue, as in the Orchideæ.

These peculiarities have been more or less fully described by R. Brown (Organs and Mode of Fecundation in Orchideæ and Asclepiadeæ ; Linn. Trans. 18383, p. 710), and recently by Meyen (Physiologie, &c., Bd. I, p. 207). A phenomenon, however, has escaped both of these most acute observers, which I am notwithstanding disposed to regard as one of the most essential. In very large and beautifully developed cytoblasts, for example, in the recently formed albumen of Phormium tenax and Chamedorea schiedeana (pl. I, fig. 5), there is observed (whether sunk in the interior or on its surface, is not yet clear to me) a small, sharply defined body, which, judging from the shadow that it casts, appears to represent a thick ring, or a thick-walled hollow globule. In examples which are not so well developed, only the external sharply defined circle of this ring can be observed, and in its centre a dark point; for example, in the stipes of the embryo of Limnanthes Douglasii, Orchis latifolia (pl. I, fig. 21), Pimelea drupacea (figs. 14, 15).