CELL-CONTENTS AND FORMS OF CELLS. 277 



tudinal walls. When the activities of plants are suspended during 

 the winter, there is formed on either side of the sieve plates a layer 

 of a colorless, mucilaginous substance, known as callus, which has 

 somewhat the appearance of collenchyma, but is colored brownish 

 by chlor-zinc iodide. 



The sieve cells contain an albuminous substance somewhat 

 resembling protoplasm ; in some instances starch grains have also 

 been found. 



When the activities of the sieve tubes have ceased, they be- 

 come altered in shape, and are then known as altered sieve. In 

 the drying of plants a similar alteration is produced, and the 

 sieve of vegetable drugs is referred to as ' obliterated ' sieve. 



Protecting cells include those cells which are located on the 

 outer parts of the plant. The function of these cells is to lessen 

 the rate of transpiration, or the giving off of water; to furnish 

 protection against changes of temperature, and to protect the 

 inner tissues against the attack of fungi and insects ; they also have 

 a mechanical function (Figs. 147, 157). 



Depending principally upon their composition, these cells may 

 be divided into two classes, namely, epidermal cells and cork cells. 



The epidermal cells constitute the outermost layer of the 

 plant. They contain cytoplasm, but the plastids in some instances 

 are wanting; in petals, etc., they also contain dissolved color- 

 ing principles ; and on account of the relatively large amount of 

 water which they contain they are classed among the important 

 water-reservoirs of the plant. 



The outer walls are principally characterized by one or more 

 lamellae of cutin, these uniting to form a continuous wall. The 

 cutin is often associated with wax, this constituting the bloom of 

 fruits ; less frequently such inorganic substances as calcium car- 

 bonate, calcium oxalate, and silica are present, and not infre- 

 quently mucilage is present, as in the walls of certain seeds (Fig. 

 119, A). 



On surface view the form of these cells varies from nearly 

 isodiametric to oblong; they may also be polygonal or branched. 

 In transverse section their radial diameter is much the shorter. 

 In some instances the inner and side walls are considerably thick- 

 ened, as in the seeds of a number of the Solanacese (Fig. 136, A). 



