77/ A' NUCLEUS 



61 



Fig. 16. 



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



ing a considerable amount of colorless cytoplasm. lis 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 A The thickening of 

 cells (Fig. 16, A) it frequently, though the inner wall of an epi- 



. ,. .i n dermal cell of Scopolia 



not always, lies near the wall upon atropoides . H origin of 



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



deposited. This relation of position to ' fter a l ' 



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, Bhowing irregular form 

 and finely divided state of the chromatin. {After Korschelt, L896.) />. Spirostomum 

 ambiguum, with moniliform nucleus. (After Shin.) C, Nucleus from Balivary 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 Oruber, 1884.) /••'. Nucleus from root tip of Afar- 

 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 surface is considerably increased (Fig. 17, A). 

 Nuclei seem rather commonly to undergo amoeboid changes in shape; 



