134 THE MICROSCOPIST. 



10. Gum is a viscid secretion. What is called gum 

 tragacanth, is said to be partially decomposed cell-mem- 

 brane, and is allied to amyloid matter. 



Forms of Vegetable Cells. From the account given in 

 the chapter on biology, page 123, it is evident that the 

 form of cells is quite varied, and often depends on the 

 amount of pressure from aggregation, yet function also 

 has much to do in the determination of shape. Thus 

 while most elongated cells are lengthened in the direction 

 of plant-growth, in which is least resistance, the medul- 

 lary rays of Exogenous stems are elongated in a horizon- 

 tal direction. Some cells are cubical, as in the leaves of 

 the yellow water-lily, Nuphar lutea (Plate VII, Fig. 101). 

 Others are stellate, as in the rush (Plate VIII, Fig. 102). 

 In many tissues are large cavities or air-chambers alto- 

 gether void of cells, and in leaves such cavities communi- 

 cate with the external air by means of stomata or pores 

 (Plate VIII, Fig. 103), which are usually provided with 

 peculiar cells for contracting or widening the orifice. 



The Botanical Arrangement of Plants. Considered with 

 reference to their general structure, plants are divided by 

 botanists into cellular and vascular. The first of these 

 classes is of greatest interest to the microscopist, as em- 

 bracing the minuter forms of vegetable life. 



The classification and natural grouping of plants is yet 

 far from being perfect, although microscopic examinations 

 have largely contributed to an orderly arrangement of the 

 multitudinous varieties in this field of research. In the 

 present work we propose only a brief outline of typical 

 subjects of interest, with the methods of microscopic ex- 

 amination. 



Fungi. At page 127 it was stated that all living beings 

 may be grouped in three divisions, fungi, plants, and 

 animals. Botanists generally class fungi among cellular 

 flowerless plants. They cannot assimilate inorganic food 

 as other plants, but live upon the substance of animal or 



