50 



INTRODUCTION TO CYTOLOGY 



Fig. 21. — Position of 

 nuclei in differentiating 

 cells. A, thickening of 

 inner wall of epidermal 

 cell of Scopolia. B, origin 

 of root hairs in Pisum. 

 {After Haberlandt.) 



it lies in the region characterized by the most active metabohsm.^ For 

 example, in young, growing root hairs and pollen tubes it is commonly 

 found a short distance from the elongating tip. Wendel (1918) observed 

 that the root hairs of Sinapis alba seedlings grow first at the apex, then at 

 the base, and sometimes at the apex once more, and that the position of 

 the nucleus changes accordingly. In thickening epidermal cells (Fig. 21) 

 it frequently, though not always, lies near the wall upon which the thick- 

 ening material is being deposited. 



In form the nucleus is typically spherical or ellipsoidal, its shape, 

 like its position, being determined by a number of physical factors. 



Under comparatively uniform condi- 

 tions, as where a small nucleus lies in 

 a relatively large amount of non- 

 vacuolate cytoplasm, a spherical shape 

 is ordinarily assumed. Exceptions are 

 often seen in cells with specialized func- 

 tions. In the cells of the spinning glands 

 of Pieris and Vanessa (butterflies) the 

 physiological conditions lead to the as- 

 sumption of very irregular forms w^here- 

 by the nuclear surface is considerably 

 increased (Fig. 22). Analogous changes 

 occur in Lilium bulb cells (Goldstein, 

 1928). Nuclei seem rather commonly to 

 undergo amoeboid changes in shape, such 

 active movement being directly observa- 

 ble in the nucleus of the living cycad spermatozoid. In the long, narrow 

 cells of vascular bundles the nuclei, which are not free to grow in all 

 directions, come to be correspondingly elongated. In Stentor and 

 Spirostomum the nucleus has the form of a string of beads (Fig. 22, B).^ 

 Nuclei show a wide variation in size, ranging in plants from extremely 

 minute nuclei like those of Mucor, 1/x or less in diameter, to the relatively 

 gigantic nucleus of the Dioon egg, with a diameter of 600;u. A similar 

 range is seen in animal nuclei. Although the nuclei of the fungi are 

 characterized by small size, most of them being less than 5jLt in diameter, 

 they may grow to be very large at certain stages. The primary nucleus 

 of Synchytrium, for instance, reaches a diameter of over 60/x. The major- 

 ity of nuclei, however, fall between 5 and 25ju. In spite of the wide range 

 in the size of nuclei of different organisms, it is generally uniform in a 



^ This relation of position to function was emphasized in the works of Haberlandt 

 (1887) and Gerassimow (1890, 1899, 1901). 



8 A discussion of the various factors influencing nuclear shape is given by Champy 

 and Carleton (1921). Tischler (1921-1922) and Goldstein (1928) describe many 

 unusual forms of nuclei. Tischler gives a long list of measuraments of plant nuclei. 



