140 THE ELEMENTS OF VEGETABLE HISTOLOGY 



that the secretion remains within the individual cell 

 and is not stored in a special cavity (Plate 50, No. 1). 

 Secretion cells may show but little differences in size 

 and form from the surrounding cells, except as the 

 secretion displaces the protoplasmic contents or 

 accumulates and distends the cell walls. In certain 

 types of unicellular secretion cells, the secreted prod- 

 ucts are apparently stored within sacs especially 

 formed for the purpose. The materials produced by 

 these cells include oils and oleoresins. 



Vittae. — Oil channels or vittae are especially char- 

 acteristic of the Umbelliferous fruits. They are located 

 in the fruit parenchyma or mesocarp and appear as 

 irregular duct-like structures extending the full length 

 of the fruit. (Plate 50, No. 2). The number of 

 vittae present is a diagnostic character of interest in 

 the differentiation of closely related species. Vol- 

 atile oils are the most important products secreted 

 by these oil ducts. 



STORAGE TISSUES 



The period of greatest activity in the manufacture 

 of nutrients is the summer season. Perennial plants 

 must lay by a store of food to carry them over the 

 winter months in which the production of nutrients 

 practically ceases. The tissues concerned in storing 

 nutrients include parenchyma cells, secretion cavities, 

 collenchyma cell walls and the cavities of fibers and 

 stone cells. 



Parenchyma. — The parenchyma cells of the cortical 

 and pith regions are the chief storage places for plant 

 nutrients. Mention has already been made of the 

 formation of starch in the parenchyma cells by the 



