THE NUCLEUS 



FIG. 16. 



A, The thickening of 

 the inner wall of an epi- 



o 



at least partially, in the chloroplast, and this is true even in cells possess- 

 ing a considerable amount of colorless cytoplasm. Its position is also 

 related to the functions of the cell: in 

 general it lies in the region character- 

 ized by the most active metabolism. 

 For example, in young growing root 

 hairs (Fig. 16, B} and pollen tubes it 

 is commonly found where elongation is 

 taking place, and in thickening epidermal 

 cells (Fig. 16, A) it frequently, though 

 not always, lies near the wall upon 



which the thickening material is being root hairs in Pisum sativum. 

 deposited. This relation of position to (After Haberland ^ 

 function was emphasized in the works 



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

 In form the nucleus is typically spherical or ellipsoidal, its 

 shape being determined by a number of physical factors. 

 Under comparatively uniform conditions, as obtain where a small 

 nucleus lies in a relatively large amount of non-vacuolated cytoplasm, 

 a spherical, shape is assumed because of the phenomena of surface 

 tension. Exceptions are often seen in cells with specialized functions. 



FIG. 17. Unusual forms of nuclei. 



A, portion of nucleus from spinning gland of Vanessa urticce, showing irregular form 

 and finely divided state of the chromatin. (After Korschelt, 1896.) B, Spirostomum 

 ambiguum, with moniliform nucleus. (After Stein.) C, Nucleus from salivary gland of 

 Chironomus: the chromatic material exists as a series of discs in a convoluted thread, which 

 ends in two nucleoli. (After Balbiani, 1881.) D, Choenia teres, with chromatic granules 

 scattered throughout the body. (After Gruber, 1884.) E, Nucleus from root tip of Mar- 

 silia, showing concentration of chromatic material in the nucleolus. (After Berghs, 1909.) 



In the cells of the spinning glands of Pieris and Vanessa (butterflies) 

 the physiological conditions result in the assumption of very irregular 

 forms whereby the nuclear surf ace is considerably increased (Fig. 17, A). 

 Nuclei seem rather commonly to undergo amo?boid changes in shape; 



