374 VEGETABLE GROWTH. 



one of two classes : either thej^ are capable of producing other 

 cells, or, incapable of this, they develop into cells for some 

 special offlee. To the former class belong all merismatic tissues ; 

 (see 201) from the latter all the permanent tissues are derived. 

 Since growing cells have such different destinies, we must ex- 

 amine them in their earliest stage to find what they have in 

 common. 



993. The simplicity of structure in many of the lower plants is 

 so great that a living cell can be kept under observation through- 

 out its various stages, and through its transparent wall all the 

 changes which go on withiu it can be noted. But the points of 

 growth in most plants, especially those of the higher grade, are 

 hidden bj- more superficial cells ; and upon removal of these pro- 

 tecting parts, pathological changes are brought about at once, 

 from exposure and mechanical injurj', and health}^ growth is 

 arrested. In a few instances onh', such as plant-hairs and 

 other epidermal structures, is it possible to observe directly the 

 progress of cell-division. Growth in deeper parts must be ex- 

 amined by an indirect method ; that is, like parts must be com- 

 pared at different stages of development, care being taken to 

 select those which have been kept under nearly the same ex- 

 ternal conditions. B}' judicious selection of material for the 

 examination of growth, specimens can be found which exhibit 

 in a single section several different phases of cell-division. 



994. When fresh material is employed, the sections are so 

 much distorted that it is diflScult to secure satisfactory results ; 

 in fact, the discordant views relative to the formation of cells are 

 largely attributable to this source of error. If, however, the 

 tissue to be examined is placed for a while in absolute alcohol, 

 either with or without a little chromic acid, the cell-wall is 

 rendered so much harder that the sections are not seriously 

 distorted, and the contents of the cells are more clearlj' seen. 

 AVhen the treatment is supplemented bj- the use of staining 

 agents adapted to special cases, the course of development ot 

 new cells can be followed out with comparative certainty. 



996. In the protoplasm of nearly all vegetable cells there is a 

 spheroidal or lenticular body apparently denser than the proto- 

 plasm itself. It retains the name nucleus, given to it b}' Eobert 

 Bi'own, who first called attention to its importance. Under ordi- 

 nary circumstances it can readily be detected in all active cells 

 of the higher plants. 



When living, it resists, like the protoplasm in which it is 

 embedded, the entrance of all coloring agents ; but when dead it 



