26 INTRODUCTION. 



small ones, at corresponding stages of mitosis. The presence or ab- 

 sence of extra-nuclear nucleoli may not depend so much upon the 

 plant, perhaps, as upon the condition or activity of the cell. From 

 the spindle stage of the first to the end of the second division there is 

 no noticeable regularity in the behavior of these bodies. In different 

 cells in the same stage of mitosis they may be present or wholly want- 

 ing. Even after the daughter nuclei are provided with membranes, 

 and a nucleolus is present in each, extra-nuclear nucleoli are to be fre- 

 quently seen in the cytoplasm. The same holds also for the second 

 mitosis. A careful investigation of the behavior of the nucleolus in 

 both Thallophyta and higher plants has shown that the nucleolus 

 appearing in the daughter nucleus is not one of the extra-nuclear 

 nucleoli which happened to lie near the chromatin, or in such a posi- 

 tion as to be included by the nuclear membrane, but that the nucleolus 

 arises anew in each daughter nucleus. The nucleolus appearing in 

 the daughter nucleus arises usually near or in contact with the chro- 

 matin thread, but it is not implied that the nucleolus represents reserve 

 chromatin. 



In the higher plants and in those with typical nuclei the morpho- 

 logical evidence furnished by a study of karyokinesis, as well as the 

 evidence of experimental physiology, goes to show that the nucleolus 

 in such plant cells represents so much food material which can be 

 drawn upon by the cell according to its needs. Whenever the activity 

 of the cell is more intense, the nucleolar substance tends to become 

 diminished, and it matters not whether the activity is directed toward 

 constructive work or the production of energy. It is true that in some 

 cases the food material furnished by the nucleolus seems to be used in 

 a large measure by the chromatin, for example, in Dictyota, but in 

 others by other parts of the living substance, as in the growth of the 

 spindle or cell plate. In certain species of Spirogyra (Wisselingh, 

 '98), in which, as it has been claimed by several investigators, the 

 nucleolus furnishes directly one or more chromosomes, greater diffi- 

 culties present themselves. It is not improbable that the nucleolus of 

 such plants as Spirogyra may possess a totally different composition 

 from that of the typical nucleolus, and we may, therefore, speak with 

 propriety of chromatht nucleoli. However the behavior of the 

 nucleolus is not w 7 ell enough known in the plant kingdom to justify 

 any attempt to harmonize all the facts now known. Applied to the 

 higher plants the above conclusion seems to be very reasonable, since 

 the facts there are almost wholly confirmatory. 



