528 



KNOWLEDGE & SCIENTIFIC NEWS. 



[September, 1906. 



process of karyokincsis; and vvc see that it is a method 

 whereby a very accurate redistribution of chromatin 

 taivCS phicc for the formation of equal daug;hter-nuclei 

 in each of tiie freshly formed cells. Tlic process has 

 been gone into somewhat fully on account of its im- 

 portance. \Vc pointed out some way back the fact that 

 economy was carefully attended to in the higher plants, 

 and we have here a means whereby each fresh cell is 

 appointed equal quantities of nuclear substance, so that 

 each cell has equal chances of still further division, 

 !.,Maiited the necessary conditions of an adequate supply 

 of food-materials, including water and oxygen. We 

 sec, moreover, what an important structure the nucleus 

 is in the case of these cells of the higher plants; 

 whereas, in those lower plants in which cell-division 

 takes place without mitosis, the nucleus seems to take a 

 less important place, although it may still possess a 

 very definite function as regulator of nutrition or a 

 cell-centre of some sort. 



4. — The further modification of a cell, as dependent 

 upon its position, and the ultimate function it has to 

 fulfil. 



The several tissues of which a [ilant is made up, such 

 as the ass;milatory, conducting, and reprtiductive, are 

 characterised to a certain extent by the form and func- 

 tions of their component cell-units. It is not intended 

 here to go into a detailed description of the micro- 

 scopical features presented by these various types, but 

 rather to try and elucidate some of the causes which 

 d;.'termine the wide divergencies from the simple form 

 which can be effected in a living cell. 



The mass of embryonic tissue in a young bud, or at 

 the apex of a rapidly growing stem, is mainly composed 

 of the kind of cell we started by examining in this 

 article, t.c, one in which we can recognise cell-wall, 

 protoplasm filling the cell-cavity, nucleus and plastids. 

 l^acli of these cells is capable of active division, and, 

 up to a certain point, maintains its simple character- 

 istics. But a little way distant from tlie apex, towards 

 the central pai'ts of the young tissue — in other words, 

 the outer boundary of that portion of the young axis 

 known as tiie " plerome " — we find certain of these cells 

 undergoing elongation, thickening of their cell-walls 

 with characteristic markings on them, and a gradual 

 diminution in protoplasm and other cell-contents. The 

 ci'lls, in fact, are becoming slowly con\erted intO' vas- 

 cular elements continuous with those of the main stem, 

 and the causes determining this transformation are 

 partly dependent upon the position of these cells, partly 

 upon the nature of the food-materials supplied, and in 

 part, perhaps, upon a certain inherent capacity 

 possessed by the protoplasm in these regions. That 

 this capacity in any given part admits of modification, 

 is seen in the case of a formation of adventitious organs 

 (such as roots) from cuttings taken from species of 

 begonia and other plants, an entire plant being in this 

 way reproduced; and in cases like this we see the effect 

 which sudden change has upon the various parts of a 

 plant, and the great adaptability of protoplasm to 

 altered conditions of existence. Position is, to some 

 extent, a factor in the determination of function, as a 

 given cell is surrounded bv others of the same tissue, 

 which can influence, and perhaps regulate, its meta- 

 bolism. But there are a few apparent exceptions to 

 this, as in the case of cells known as " idioblasts " — that 

 is, cells more or less different in structure and functions 

 from those by which they are surrounded. Such are 

 tannin-cells, cells of oil-glands, and those in which large 

 masses of crystals sometimes separate out. It would 



seem here as if the protoplasm of a given cell were 

 capable of modifying its action in such a way as to 

 take on the additional or sole function of manufactur- 

 ing substances, which may or may not be of future 

 u.se to the plant; but as to why protoplasmic action 

 should in these cases be modified is difficult to explain, 

 and explanation would only become possible on the 

 theory of the mutual inter-dependence of cells. 



In the embryo-plant, certain rudimentary tissues are 

 laid down, such as those composing radicle, young 

 stem, and leaf rudiments; there are grounds for as- 

 suming that the formation of these embryonic organs 

 is governed to a certain extent by a hereditary in- 

 fluence transmitted with the protoplasm, from which 

 the cells composing these organs are ultimately pro- 

 duced. Later on, when external surrounding condi- 

 tions begin to act upon these cells, we find that the 

 hereditary influence loses to a certain extent its value, 

 and the further modification of the cell depends more 

 and more upon these conditions and upon the final posi- 

 tion relative to other tissues which it takes up. 



We find in some cases that hereditary characters are 

 retained up to a certain point, but that these characters 

 tend slowly to become modified, even to complete ab- 

 sence, by the predominant effect of other conditions; 

 such an instance we find in prefertilisation stages in the 

 embryo-sac of the Angiosperms, where the antipodal 

 cells formed during the divisions of the nucleus of the 

 embryo-sac represent a former prothallium, thus 

 showing to a certain degree a relationship of the 

 Angiosperms with the Gymnosperms, and Cryptogams. 

 But the cells of this prothallium are now so reduced 

 and few in number that, in time, they will probably be 

 non-existent. 



We cite this case simply to show that hereditary in- 

 fluences are at work during embryonic stages, and that 

 we must not lose sight of this possibly important factor 

 in cell-modification. The cell, however, is modified in 

 the various parts of a plant in order to be able to fulfil 

 certain functions, and this change is completed when 

 the various organs have, under the influence of external 

 conditions, attained their perfection. As to the causes 

 of this modification, we have seen that some may have 

 their origin in a certain hereditary capacity impressed 

 upon the cells of an embryo^plant, and yet others are 

 certainly due to the influence exerted by surrounding 

 conditions. Some remanent hereditary capacity must 

 likewise be assumed to be present even in the proto^ 

 plasm of cells of even highly developed organs, so that 

 we must not think subsequent modification during 

 growth entirely due to external conditions. 



Moreover, it is important to remember that the adapt- 

 ability of protoplasm is limited to young cells, for a 

 cell that has been already modified to a certain extent 

 will probably, even if placed under entirely new condi- 

 tions, proceed upon the same lines of development as 

 before, or else be unable to exist at all under these 

 altered conditions. 



Having now examined the cell under the four head- 

 ings set out at the beginning of this article, we must 

 further try and gain an idea of the general working 

 !/f the cell looked at from the point of view of a unit 

 depending upon the existence of other cells, as well as 

 upon its own labours. By means of the various tissues 

 into which a plant may be divided, a very accurate 

 division of labour is effected; some of the work done 

 by living cells is manifested in the formation of units 

 that soon become devoid of protoplasm, such as in the 

 vood of stem and root, but these elements have a very 



