18 ORGANOGRAPHY. 



of the reagents applied, is by no means clear, for when a 

 cell containing protoplasm is examined Avithout the aid of 

 reagents, no membrane thus boimding the contents of the cell 

 can be clearly distinguished; indeed, it would appear far more 

 probable that this so-called primordial utricle is merely a thick 

 ened layer of the protoplasm lining the cellulose-wall, which 

 assumes the character of a true membrane under the action of 

 reagents, just in the same way as any thickened gelatinous 

 matter, when exposed to the air, would become invested as it 

 dried up by a more or less evident pellicle. This thickened 

 layer of protoplasm or primordial utricle, performs, as we shall 

 afterwards find, a very important part in the process of cell- 

 development ; it may be, therefore, always observed in young 

 and vitally active cells. Its existence is in most cases but 

 transitory, disappearing when the secondary layers are being 

 deposited. It is, however, a permanent formation in cells con- 

 taining chlorophyll (the green colouring matter of plants), 

 as, for instance, in the cells of leaves, and in many of the lower 

 kinds of plants. 



Nucleus or Cytoblast. — Almost all young cells contain one 

 or more bodies called Nuclei or Cytoblasts, Avhich are always 

 in intimate connection with the Primordial Utricle. In the 

 cells of the higher classes of plants the nucleus consists of a 

 rounded or lenticular gTanular-looking body {figs. 8 and 

 35) generally more transparent than the protoplasm in which 

 -,. it is placed, and containing ahnost invariably 



^^' ■ in its interior, one or more usually sharply 



defined bright points called Nucleoli. The 

 nucleoli vary somewhat in their appearance; 

 more commonly they seem to be formed of 

 solid transparent granules, Avhilc in other 

 cases they appear like small cavities in the 

 interior of the nucleus. The nucleus and the 

 nucleolus are best observed under the action 

 of iodine, which colours them yellow or 



Fig. 35. Cell with brownish. The size of the nucleus in pro- 

 nucleus and nu- ^. ^ ^, .^ ^ ^, ,, . *^, 

 cieoius. portion to the cavity or tlie cell varies greatly; 



in the very young cells of newly formed parts 



the nuclei occupy nearly, or entirely, the whole cavity; while in 



the cells of other parts they are of but small size in proportion 



to the cavities. 



There are two theories as to the nature of nuclei ; by most 

 observers they are considered to be solid granular structures 

 as described above ; while Niigeli and otlicrs regard them as 

 vesicles. 



The above remarks as to the ap])earanco and nature of nuclei 

 apply more esj)ociaIly to tliosc found in the cells of the higher 

 plants. In those of the lower jilants they are frequently very 



