STRUCTURE OF PLANTS. 11 



just mentioned experience sensations, nor that the motions per- 

 formed by them are directed by will : sometimes these move- 

 ments result from the action of heat or humidity upon certain 

 parts of their tissues, and at other times they can only be com- 

 pared to the automatic movements, which are readily brought 

 about by means of electricity or galvanism, in animals that have 

 been recently killed and deprived of the functions of relation. 



Of the Structure of Plants in General. 



9. Although plants differ very widely from each other in their 

 external forms, they closely resemble each other in the materials 

 of which their organs are composed : if we examine the internal 

 structure of plants by the aid of a microscope, we find they con- 

 sist entirely of cellular tissue alone, or at most of cellular tissue 

 united to vessels. 



10. Plants that are composed entirely of cellular tissue are 

 called cellular plants, and those formed of cellular tissue and 

 vessels are named vascular plants. 



Of Cellular Tissue. 



11. The cellular or utricular tissue of vege- 

 tables consists of a multitude of vesicles (minute 

 cells) filled with a liquid or -other substance; 

 sometimes these little bladders are rounded and 

 loosely attached to each other (fig. 2); but in 

 general they are so strongly pressed against each Fl 8- 

 other that they are flattened at the points where VESICLES. 



they touch, and take the form of polygons (from 

 the Greek, polus, many, and gone, sides, jigs. 3 

 and 6, g, c) ; at the same time their union be- 

 comes so intimate that it is difficult to separate 

 them, and the cells formed by their cavities seem 

 to be separated only by simple partitions, as cavi- 

 ties would be if hollowed out of a continuous or 

 solid mass, like the cells of a honey-comb, for example. 



Explanation of Fig. 2. Utricula or cells of the cellular tissue, which 

 have preserved their primitive form, magnified. 



Explanation of Fig. 3. The same cells which have become poly'gonal 

 in consequence of pressing against each other. 



9. Do plants differ from each other in their internal structure as much as 

 they do in their external form ? 



10. What are cellular plants ? What are vascular plants ? 



11. Of what does the cellular tissue of plants consist? Are all cells of 

 the same form ? 



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