ELEaiEXTART STRUCTURE. 27 



perceptibly overlapping, they are called parenchyviatous ; but 

 that when pointed at their ends, so that in combination they 

 overlap each other, they are termed prnsenchyniatous. We have 

 also seen that such extreme forms are connected by all sorts of 

 transitional ones. Formerly, all elongated organs found in 

 plants were supposed to have an entirely distinct origin from 

 the cells, and were described under the names of Woody Fibres, 

 and Vessels or Ducts, but it is now known that they are all 

 derived originally from ordinary cells, and owe their pecu- 

 liar appearances, either to various modifications in shape, which 

 the latter undergo in the course of growth, or to their combi- 

 nation and union with each other. This common origin of the 

 Woody Fibres of old authors and the Vessels or Ducts, with the 

 cells, is proved by the fact, that gradual transitional forms from 

 the one to the other may be commonly obseiwed; and also by 

 tracing their development, when it will be found that all these 

 organs, however modified in shape and appearance, are derived 

 originally from the ordinary cell. All the observations made 

 previously, therefore, as to the chemical and general properties 

 of cell-membrane, as well as to the mode of growth and depo- 

 sition of secondary layers, apply equally to the Vessels or 

 Ducts. We have already stated this to be the case with regard 

 to the Woody Fibres, Avhich we have spoken of under the name 

 of AVood-cells. By the combination of the different kinds of 

 cells, we have various compound structures formed which are 

 called Tissues, these we now proceed to describe. The most 

 important and the most abundant of them all is 



1 . Parenchyma. — This, which may be considered as the typical 

 form of cellular tissue, is composed of comparatively thin-walled 

 cells, whose length does not exceed their breadth, or in which the 

 proportion of the two diameters does not vary to any remarkable 

 extent. Parenchyma has been divided in various ways by 

 authors, the divisions being founded upon the forms of the 

 component cells, their modes of cohesion, and other pecu- 

 liarities. It will be sufficient for our purpose to adopt the simple, 

 arrangement of Schleiden, which is as follows: — 



1. Incomplete Parenchyma is that in which the component 

 cells are in contact only at a few points, so that numerous 

 interspaces are left between their sides ; of this there are 

 two varieties. 



a. Round or elliptical Parenchyma. {Fig. 50.) — This is formed 

 of rounded or more or less oval cells, and commonly occurs in 

 succulent plants, and in those parts where the tissues ai'e of a 

 lax nature, as in the pulpy portions of leaves and fruits. The 

 name of merenchyma is frequently applied by authors to this 

 form of parenchyma. It is connected by various transi- 

 tional forms with 



