90 THE PLANT CELL. 



may be met with, and the cells composing the other tissues of 

 these plants, although, as a rule, simple in type, have some 

 resemblance to those met with in Higher Plants. It will, how- 

 ever, be seen that in plants below the Fungi — viz., the Algae — 

 cells are often met with which in all respects agree with the 

 typical assimilating cell, which was studied in Chapter ii. 



It is not intended here to pursue the study of the cells of 

 Fungi any further, but to proceed to the examination of a few 

 of the more well-defined types of lower plant organism met 

 with amongst that group of the Thallophyta known as the Algse. 



B. Cells occurring amongst the Algae. 

 The types here selected for study will be : — 



a. Spirogyra (belonging to the Conjugatce). 



b. Vaucheria (belonging to the Siphunece). 



c. Sphserella (belonging to the Volvoeinem). 



d. Melosira (belonging to the DiatomacecB). 



a. Spirogyra. — This plant is one of the filamentous Algae, in 

 which a large number of cylindrical cells are united end to end 

 to form a colony. It is found at the bottom of ponds in 

 the form of large interwoven masses of a light green colour. 

 Each of the cells composing a filament is relatively a large 

 one, and, when examined microscopically, may be seen to be 

 composed of the following parts (see Fig. 72, 1) : — 



i. Externally, a delicate CBll-Wall. 



ii. An internal layer of cytoplasm, lining the inner surface of the 

 cell- wall and enclosing; a central vacuole ; from this layer bridles pass 

 to a central mass in which lies 



iii. The nucleus. This body possesses a well-defined central spot, the 

 nucleolus. 



iv. A spirally wound ribbon-shaped ChlOPOphyll band, which lies 

 next the cell-wall embedded in the peripheral cytoplasm. The edges of 

 the band have a sinuous appearance, and the axial portion seems to be 

 rather thicker and more refractile than the lateral parts. Arranged at 

 regular intervals along the axial portion of the band are to be seen rounded 

 refringent structures, which are known as the pyrenoids ; these, as will 

 soon be seen, are active StaPCh-formerS and StOPePS, and require 

 special examination. For this purpose a fresh preparation of a filament 

 may be treated with a drop of weak iodine solution, when the following 

 effects may be noted : — 



