5i6 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1906. 



unit capable of working for itself, but also as being 

 dependent upon others by reason of intcrprotoplasmic 

 connections, we ha\e several instances of the value of 

 this conception, especially with regard to certain move- 

 ments that are produced by tactile stimuli in plants 

 such as the Sundew and Mimosa. In the former, con- 

 tact of a small foreign body (preferably of an organic 

 nature) with the glandular outgrowths on the upper 

 surfaces of the loaves leads to a bending o\er of these 

 outgrowths (h.airs) so as finally to enclose the irritating 

 substance. It has been shown that here the stimulus 

 passes most probably by certain living elements of the 

 central tissue of the excrescence, and that this stimulus 

 finally influences cells at the base of the hair, which in 

 turn alter their turgidity on one side of the hair more 

 than on the other. It is, however, to the protoplasm 

 that the effect is ultimately due, and it is upon this 

 substance that the stimulus has effect. A similar 

 result may be observed in the leaves of the Mimosa 

 pudica, where a tactile stimulus, the rate of travel of 

 which can be measured, produces a depression of, first, 

 the leaflets and then the whole leaf, consequent on 

 diminished turgidity in the cells of the pulvini situated 

 at the base of the petioles. 



In some of the lov^er plants, Spirogyra, for instance, 

 it has been found possible, by suitably altering the 

 surrounding conditions, to substitute amitotic for 

 mitotic division; and it is to be assumed that in higher 

 plants, mitosis has arisen in order to cope with altered 

 conditions of existence, for we shall sec that economy 

 is of the bighe,st importance to the cell-community, and 

 mitosis affords an almost exactly equal distribution of 

 nuclear .substance in each of the two d.iughter nuclei. 

 Moreover, in the higher plants, it is probably a very 

 fixed and constant method of division, not readily 

 changed to the amitotic form; and we shall here study 

 mitosis as it occurs in the Gymosperms and Angio- 

 sperms. [In the following figures, the centrospheres 

 have been omitted, as the methods of preparation of 

 the sections were inadequate to show them, even as- 

 suming that they were pre.sent at all.] 



In order to follow the karyokinetic changes properly, 

 it is necessary to study, first of all, the minute struc- 

 ture of the resting nucleus. This body, as we have 

 seen, is supposed by some to pxassess a definite nuclear 

 membrane, which encloses at least three different por- 

 tions, named respectively, nuclear plasm, nuclear net- 

 work, and nucleoli. The network is composed of a 



We have now brieflv considered the effect of some 

 of the commoner stimuli upon the protoplasm. The 

 manner in which the latter responds is not the same in 

 all cases, and in some parts of a plant a given stimulus 

 might produce an effect which it would not call forth 

 in a more remote part, and vice versa. Tliere is, how- 

 ever, a certain " specific irritability " inherent in tlie 

 protoplasm of various parts of a plant which enables 

 it to make use of one or perhajis several stimuli in ex- 

 cess of all others, but what this is due to is not known. 

 Perhaps heredity has some value in the explanation 

 of this curious, yet undeniable fact; and we shall see 

 later the effect of "position " upon the cells of any 

 part of a plant. 



3. — The reproduction of similar cells from pre-exist- 

 ing ones. 



In the higher plants, cell-division is always preceded 

 by nuclear-division, or karyokinesis; in some instances, 

 however, the formation of definite cell-walls does not 

 immediately follow division of the nucleus, notably in 

 the process known as free cell-formation, where a large 

 number of nuclei are first formed, Iving free in a shell 

 of f)eripheral protoplasm in the embrvo-sac. 



In a few cases, the nucleus is capable of " amitotic " 

 division, that is, simple fission into two more or less 

 equal portions, but in this case, no fresh cells are 

 formed, and the nuclei simply increase in number. 



substance known as linin. which is, according to 

 various observers, unstainable by logwood, and on the 

 meshes of this network are situated at certain points, 

 small masses of a substance known chemicallv as 

 nuclein, or chromatin, which stains deeply with log- 

 wood, and the more so the nearer the time for division 

 approaches. Tlie nucleoli (one or more, as the case 

 may be) form well-marked masses at certain points of 

 the network, but are usually quite distinct from this 

 latter; they stain with logwood, but show some differ- 

 ences when the nuclei are treated with a compound 

 stain, so that nucleoli are stained differently to the 

 other nuclear structures. 



If centrospheres are present, they will be seen close 

 to the nucleus, in the surrounding protoplasm (kino- 

 plasm); but it is not at all certain whether these bodies 

 are present near the resting nucleus or whether they 

 are rather to be looked upon as originating from a 

 nuclear structure about the time that karyokinesis is 

 about to start. If we examine a longitudinal section of 

 a rapidly growing root-tip, in which the nuclei are of a 

 good size (hyacinth roots serve excellently), and in 

 which the various staining processes and sub.sequent 

 preparation have been carefully carried out, we shall 

 find instances in different cells of all stages in mitosis. 

 The earlier stages are easily made out from the very 

 different staining reaction shown by the masses of 



