MOULDS. 167 



history of higher fungi, occur in the form of vigorous growths 

 easily visible to the naked eye. Each species has a very 

 characteristic appearance, which is due, not only to its struc- 

 ture, but also to the colour which it affects at a particular 

 stage of its development, varying in shade from the purest 

 white to the deepest shades of colour. 



The individual cells of which the body of the mould plant 

 is built up consist of a cell-wall or membrane, together with the 

 cell-contents, which consist essentially of protoplasm, vacuoles, 

 and various contents, of which the most important is the cell- 

 nucleus. The membrane is composed of a substance known as 

 fungus-cellulose, differing, as a rule, in its chemical reaction 

 from the cellulose of higher plants. Both inner and outer 

 surfaces are subject to gradual thickening, and with increasing 

 age become impregnated with deposits of colouring matter, 

 and incrusted with crystals, especially of calcium oxalate. 



The protoplasm (or Cytoplasm) consists of a homogeneous 

 viscid substance packed with minute granules. This con- 

 stituent of the cell determines the growth, and a part of it 

 forms a thin layer lining the inner surface of the cell- wall. 

 The protoplasm in living cells is in constant motion ; in 

 certain cases (e.g., the young sporangiophores of species of 

 Mucor), the motion is sufficiently active to permit of its obser- 

 vation through the microscope. In very young cells the 

 protoplasm occupies the entire cell space. Later on vacuoles 

 appear, and at the same time different kinds of corpuscles, 

 amongst which the " crystalloids " may be cited, consisting 

 of albuminoid substances, which may perhaps be regarded as 

 products of secretion. There are also the widely distributed 

 fatty oils and fats, which are especially abundant in the 

 reproductive organs and the resting cells. A very important 

 part of the protoplasmic contents is the nucleus, a small 

 rounded body, which, by the addition of a suitable staining 

 fluid, becomes very prominent, and under the microscope can 

 frequently be seen to include an inner and more highly coloured 

 portion, which is called a nucleolus. By using special staining 

 methods, it has been shown that the nuclei are capable both 

 of division and of fusion, processes which are directly con- 

 nected with the different stages of development which the 



