514 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1906. 



between adjacent sieve-tubes in the phloem of the 

 vascular system of higher plants. 



The cell, as it gets still older, enlarges continually 

 up to a certain point, but the protoplasm does not in- 

 crease in volume proportionately, so that ultimately we 

 have a central " vacuole " filled with cell-sap, and out- 

 side this, lining the inner surface of the cell-wall, is a 



r# 



• ^ 



1^ 



Fig. 3- 



thin layer of protoplasm, in which is somewhere en- 

 closed the nucleus. 



This peripheral layer was first described by von Mohl 

 as the " primordial utricle." It has a very important 

 function, which we shall discuss further on. 



The nucleus is a most important structure; as usuallv 

 seen, it is a clear, rounded body, with a more or less 



FiK. 4.— Younj; Cell from pith of stem of Lupulus. n. Cell-wall. ;>. 

 Protoplasm iEndoplasm . >i. Nucleus, showing nucleolus and 



central dot, the nucleolus, in its interior. When care- 

 fully stained with logwood, it is seen to have a definite 

 structure, the most frequent being a punctate or 

 reticulate appearance, which about the time of nuclear 

 division becomes very marked indeed (see I' ig. 4). 



Numerous observers have described a membrane, the 

 " nuclear membrane," as the external contour of the 

 nucleus, and the nucleolus, of which there may be more 



than one, also seems to have a vesicular structure. 

 But more recently, it has been thought that the nucleus 

 is a rounded space in the protoplasm, the margin of 

 which is formed of firmer protoplasm (altered kino- 

 plasm)* than the main mass. 



With high powers of the microscope, and very care- 

 ful preparation, it has been shown that two bodies, 

 the " centrospheres " or " blepharoplasts " exist in the 

 protoplasm very close to the nucleus. They are of im- 

 portance in nuclear division (mitosis); but they probably 

 do not exist as formed structures in the cells of plants 

 higher than the Liverworts. 



There are certain other structures present in the 

 general protoplasm of a cell; these are the " plastids " 

 or " chromoplasts," the former term being the best, 

 as they do not always contain pigment. They are, like 

 the nucleus, specialised portions of the protoplasm, and 

 function as a rule in the manufacture of starch. \'cry 

 often they may be seen grouped round the nucleus, but 

 as yet the significance of this has not been explained. 

 It is probable, however, that during active division, tlie 

 nucleus requires a large supply of carbohydrate food, 

 and in this case the nearer the plastids are the better. 

 The chlorophyll granules so often seen in the cells of 

 the green assimilating tissues are only plastids am- 

 taining the pigment chlorophyll, and are here, o( 

 course, true chromoplasts. 



A word or two requires to be said about the grouping 

 of cells into tissues. A cell equally pressed upon all 

 sides by other cells usually assumes the form of a 

 regular dodecahedron, that is, on section, the figure 

 is a hexagon. If the pressure is not even all 

 round, then certain deviations from this natural rule 

 exist, and we get on section four, five, seven, etc., sided 

 figures. The rectangular prism is also a common form 

 assumed by cells in cortical regions, where definite 

 limited layers of cells exist, and the radial and vertical 

 pressures are as a rule unequal. The natural form of 

 the hexagon is seen well in the young pith of certain 

 plants (Sambucus). 



.A.lthough the shape of the cells in any tissue is thus 

 to a large extent determined by external physical 

 causes, the growth of the cell, as a whole, is brought 

 about by the agency of the protoplasm, which takes in 

 the various food-materials and transforms them into 

 other substances, which form integral parts of the body 

 of the cell. 



We have now briefly examined the general features 

 of the living cell microscopically, and have seen that it 

 consists of several parts. Each of these parts has a 

 definite function of its owu; and the living portions 

 of the cell, i.e., protoplasm, nucleus, and plastids, are 

 intimately related to one another in a way that we 

 shall presently see. It is instructive to look upon the 

 cell as a transformer of energy, and as a utiliser of the 

 same, but it is only a limited view of the total function 

 of the organism, for, as we shall see, there exists a 

 mutual interdependence between the different li\ing 

 cells of any plant, t 



2. — The reaction of the cell to stimuli of \arious 

 kinds. 



We have spoken of a cell as a living unit, and we 

 have now to enquire into the manner in which the 

 protoplasm responds to, in the first place, nutrition, and, 



* The " kinoplasm " is thai portion of the general protoplasm 

 which lies just outside the nucleus; it is not so granular as the 

 main mass. 



+ This view is well discusLcJ in Kerner's Natural History cf 

 Plants, Vol. I, Pt. I. 



