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THE AMERICAN NATURALIST. [Vol. XXXVI II. 



effect is that of a much coiled and twisted thread whose loops are 

 united at intervals to form large and small meshes. The ground 

 substance of this thread is called linin and imbedded in it as in 

 a matrix are deeply staining granules of chromatin. Chromatin 

 is regarded as the most important substance in the nucleus, ' 

 chiefly because of its behavior during nuclear division, and in 

 critical periods of the life history of organisms as at sporogen- 

 esis, gametogenesis and -fertilization (to be described in Section 

 V). Just before nuclear division the chromatin becomes organ- 

 ized into bodies named chromosomes which are remarkably uni- 

 form in number and definite in shape for each tissue and period 

 of the plant's life. They will be discussed under " The Events 

 of Nuclear Division " (Section II), and in Sections IV and V. 



Fig. I. — The resting nucleus, a. Embryo sac of lily with linin thread and two nucleoli. 

 ^,.Root of onion large nucleolus. , c, Tetraspore of Corallina showing large chro- 

 matin body and small nucleolus. oT, Spirogyra with central body containing chroma- 

 tin, e. Chromatin on linin net work from egg of pine. After Mitzkewitsch and 

 Chamberlain. 



In the meshes of the linin network or lying freely in the 

 nuclear sap may be found one or more bodies, generally globular 

 in form, called nucleoli. (See Fig. i a and Fig. i b). The 

 nucleolus is generally regarded as a secretion of the nucleus and 

 it is quite certain that its substance is utilized just previous to 

 and during the period of nuclear division when the spindle is 

 formed. (Strasburger '95 and : 00, p. 125, and from the work of 

 others). The structure is not always homogeneous but may 

 show in the interior small vesicles or areas of a different con- 

 sistency from the periphery. There is often present also a 

 rather thick outer shell or membrane. Sometimes the chromatin 

 in the nucleus may be gathered into a globular body that resem- 

 bles superficially a nucleolus. Such chromatin bodies are gen- 



