34 BOTANY. [CHAP. i. 



be pitted when the unthickened portions are small and 

 circular; scalariform when elongated and arranged in 

 parallel series. When the internal thickening takes the 

 form of a thread-like band arranged in a spiral manner 

 we have spiral cells, and annular when the band takes 

 the form of detached rings. 



The object of all forms of secondary thickening of 

 the cell-wall is that of giving additional strength ; the 

 unthickened portions that are opposite to each other in 

 adjoining cells are left for the purpose of allowing liquids 

 and gases to pass from cell to cell, which could not take 

 place, or at all events very slowly, if the entire surface 

 of the wall was thickened. The hardness and durability 

 of wood is due to the much thickened walls of its com- 

 ponent cells, the substance of which changes its compo- 

 sition with age, and at the same time usually becomes 

 coloured. 



Protoplasm is of a semi-liquid consistency, colourless, 

 and is either homogeneous and transparent or, more 

 frequently, turbid, owing to the presence of minute 

 drops of oil, starch, etc. In young cells the protoplasm 

 with the nucleus almost completely fills the cell, and the 

 water which saturates it collects into minute drops called 

 " vacuoles." These ' ( vacuoles" eventually run together 

 and form the central sap-cavity. The power of spon- 

 taneous movement exhibited by protoplasm is often very 

 marked, especially in the lower groups of plants, as sea- 

 weeds, fungi, mosses, etc., where the male or fertilizing 

 element, called the antherozoid, consists of a very minute 

 primordial or naked cell, i.e. } not furnished with a cell- 

 wall, and provided with one or more exceedingly slender 

 hair-like continuations of its protoplasm that serve as 



