January, 1913. 



KNOWLEDGE. 



11 



and the demonstration of an internuclear connection 

 is certainly an interesting point. 



It is noteworthy that in a few instances the 

 chromatin constituent of the nucleus takes, in place 

 of separate granules, the form of a so-called 

 " permanent spireme " suspended in the nuclear 

 plasm ; the chromatin in these cases takes the shape 

 of small discs, placed end to end quite close together. 

 The permanent spireme is best seen in the cells of 

 the salivary gland of the larva of Chironomus. At 

 times, also, the chromatin granules become split into 

 two and even in the resting nucleus a spireme may 

 be seen made up of two parallel bands of granules, 

 an arrangement which is obviously of value when 

 karyokinesis takes place, since the earlier stages of 

 the prophase are eliminated. 



With regard to the changes taking place in the 

 structure of the nucleus during mitosis, reference 

 may be made to the article mentioned above (see 

 footnote on page 10) ; these changes indicate, in the 

 main, an increase in the chromatin, a relative decrease 

 in the para-chromatin (nucleolar material) and loss 

 of the nuclear membrane. The latter change is 

 important since, if the membrane persisted, the 

 movements of the chromosomes to opposite poles 

 of the cell would be prevented ; this, then, is further 

 evidence of the presence of a membrane in the rest- 

 ing condition of the nucleus. [The question of the 

 centrosome is one more directly connected with 

 nuclear division, and will not be gone into here.] 



(c) Structure of the Plastids ; Outline 



of their Function. 

 These small bodies, many of which contain pig- 

 ment (chromoplasts) and others being colourless 

 (leucoplasts) have a structure which appears to 

 repeat in some respects that of the nucleus; but 

 there is no nuclein constituent in them comparable 

 to the chromatin of the nucleus. In the main, each 

 plastid appears to possess an outer membrane, and 

 internally, either a clear fluid part, or, as some 

 observers state, a groundwork of fibrils (spongio- 

 plasm) in the meshes of which is a clear, more fluid 

 portion; it is the latter which has dissolved or 

 suspended in it the pigment, when this is present 

 (chlorophyll, xanthophyll, carotin, and so on). 



In many cases the plastids contain starch or oil in 

 their substance, this being evidence of the secretory 

 function of these bodies, and it appears probable that 

 these stored substances are formed by an actual 

 breaking down of the protoplasm of the plastid with a 

 subsequent re-construction of the starch or oil. The 

 function of the pigment chlorophyll has already been 

 touched upon (see page 8), but it should be 

 borne in mind that in the colourless plastids (leuco- 

 plasts) found in the cortical cells of subterranean 

 stem-structures and roots, the starch is formed from 

 the circulating sugar brought to the cell without the 

 intervention of the photosynthetic process, the 

 necessary energy for the secretory reactions being 

 derived from other sources than white light. 



Fats and oils occurring in the so-called elaioplasts 



are formed in much the same manner ; but, of course, 

 oxidation is at times incomplete and a hydro-carbon 

 is sometimes formed in place of a carbohydrate. 

 {d) The Vacuoles. 

 The modern conception of the vacuole has been 

 elaborated considerably of late years ; in fact, ever 

 since the importance of the biophysical phenomenon 

 of osmosis in the cell was recognised. The fluid 

 contents of the vacuole in the plant cell always possess 

 one or more of the organic acids in an amount 

 sufficient to set up osmotic currents between the 

 fluid in the vacuole and the medium surrounding 

 the cell, with the result that in many instances water 

 and salts are drawn into the vacuole according to 

 the needs of the protoplast. 



Observations upon the origin and growth of the 

 vacuole have determined the interesting fact that 

 directly a vacuole becomes visible, the fluid contents 

 become surrounded by an extremely delicate pellicle 

 of ectoplasm (hyaloplasm), and many investigators 

 have looked upon the formation of the vacuole as 

 being akin to that of a plastid— vacuoles being some- 

 times known as tonoplasts (De Vries) in that they 

 govern the relative turgidity of a cell. The pellicle of 

 ectoplasm of the vacuole, the endoplasm, and the layer 

 of outer ectoplasm of the protoplast together form a 

 membrane, which, in the opinion of most biologists 

 nowadays, is of great importance in the process of 

 absorption by the cell of water and salts in solution ; 

 the so-called "selective capacity" of a root-hair, for 

 instance, being largely due to the presence of the 

 three living layers surrounding the vacuoles. The 

 physical laws governing osmosis naturally come into 

 play during the process of absorption, and the 

 amount of saline constituents entering the cell 

 depends largely upontherelative " osmotic pressures " 

 exerted, on the one hand, by the osmotically active 

 substances (organic acids, chiefly) in the vacuoles, 

 and on the other by those outside the cell. It will 

 be seen, then, that the vacuole governs the process of 

 absorption of raw food-materials ; but it must be 

 borne in mind that osmotic processes also govern in a 

 cell-aggregate the distribution of circulating " food " 

 (elaborated food) such as sugar, soluble proteids, and 

 the amido-acids ; all these substances are attracted 

 to those living cells in need of them for purposes of 

 construction, and here again the organic acids which 

 possess a relatively high osmotic power are probably 

 the determining factors in this attraction. The two 

 processes, then, both absorption of raw food-materials 

 and circulation of elaborated food, are dependent 

 partly upon purely physical influences and partly 

 upon the regulating action of the cytoplasmic mem- 

 branes in the cell. 



The "Bioplast" or "Pangen" Hypothesis, 

 as a Biological Conception. 

 The hypothesis that the various structures met with 

 in the cell are all ultimately derived from a common 

 elementary body, the so-called " bioplast " or 

 " pangen " is one which for some time now has 

 occupied the attention of biologists. It argues that 



