FOEM AND AEEANGEMENT OK CELLS. 



either directly opposite to each other, that is, are placed at the same 

 height (fig. 18), or are alternate, from being placed at different 

 heights (fig. 19) ; cells sometimes communicate with each other later- 

 ally (fig. 20 a a). Isolated cells, as spores of sea-weeds, occasionally 

 have free filaments, or cilia (cilium, an eyelash), developed on their 

 surface. 



Fig. 17. 



Fig. 18. 



Fig. 19. 



Fig. 20. 



The simplest kinds of plants, as mushrobms and searweeds, are 

 composed entirely of cellular tissue, and are called Cellulares. The 

 pulpy and succulent parts of all plants contain much cellular tissue, 

 and the object of horticultural operations is to increase the quantity of 

 this tissue in ordinary fruits and vegetables. The pith of trees, and 

 plants during their early development are cellular; so also are cotton 

 and rice-paper. The cell may be considered as the ultimate struc- 

 tural element of all organisms. In the simplest vegetable forms, as in 

 imicellular algse, it is adequate to aU the purposes of plant life. Vital 

 operations are carried on in all plants by means of cells, the constitu- 

 tion and functions of which vary according to the nature of the plants 

 and the position in the scale of organisation which they occupy. In 

 the higher classes of plants, certain cells are concerned in the secre- 

 tion ^of organisable products, which are elaborated by others into new 

 tissues. The life of the higher species of plants results from the 

 regular action of cells, which are of unequal value as regards the for- 

 mation of new organs and new products. In cells there are observed 

 the albsorption and movements of fluids, the elaboration of these by 

 exposure to air and light, and the formation of new cells. Schacht 

 remarks that a plant is composed of one or more cells, and that it is 

 only in the lowest species that the cells are of the same value ; in other 

 words, are of the same chemical and physical nature, and of the same 

 physiological importance. Even amongst the mushroom and sea- 

 weed orders, it is only the lowest plants which have cells concerned 

 alike in the processes of vegetation and reproduction. The higher 

 plaitts of these orders are composed of parts having different values. 



Figs. 17, 18, 19. Cells united together by their extremities. Fig. 20. Elongated 



thickened cells from the root of the Date Palm, a a. Canals of communication. 



