THE CELLULAR STRUCTURE OF PLANTS 51 



Chromoplastids (Fig. 28, c) are plastids which secrete pigments 

 other than green, which tint the plastids and the organs in 

 which they are located yellow, orange, or red. Flowers and 

 fruits often owe their color to the abundant chromoplastids de- 

 veloped in the cytoplasm of their cells. Familiar examples are 

 the yellow petals of nasturtium flowers, the red color of the 

 tomato, and the orange yellow of the berry of bittersweet. 



FUNCTIONS OF CELL PARTS 



The cell wall, as has been stated, serves as a protection for 

 the delicate protoplasts and as a skeletal framework for the 

 entire plant body. The protoplasts of the plant body have been 

 shown to be connected in some plants by delicate strands of 

 protoplasm, which pass through fine pores in the separating 

 cell walls. The protoplasts in these plants are not, therefore, 

 isolated units of living substances, but are closely joined in 

 a living body of protoplasmic units. Such a continuity of the 

 protoplasts by means of connecting strands might conceivably 

 be of direct service in the passage of foods from cell to cell or 

 in establishing a nervous connection between the various tissues 

 and organs of the plant body. Both of these hypotheses have 

 been advanced by reputable botanists, but there is as yet no 

 conclusive proof that either is true. 



The cytoplasm and the nucleus undoubtedly represent a divi- 

 sion of labor in the living substance of the protoplast by which 

 it is enabled to do its complex work more satisfactorily. 



The cytoplasm is supposed to be largely concerned with the 

 building, storage, and use of foods and with the reception of im- 

 pressions (stimuli) from the outside. We have already seen that 

 the green chloroplastids are portions of the cytoplasm which 

 build starch, and we shall learn that the starch in potatoes and in 

 seeds is stored in the cytoplasm of the cells of those structures by 

 the agency of leucoplastids. It is equally evident that the cyto- 

 plasm must be the first to feel impressions from outside of the cell. 



The vacuole inclosed by the cytoplasmic sac is filled with water 

 in which various substances formed by the cytoplasm finally 



