PRINCIPAL GROUPS OF PLANTS. 41 



of which is at the apex. These threads are known as HYPH^E 

 (singular hypha). They branch and become interwoven, forming 

 a mass or mat known as the MYCELIUM (Fig. 23). The myce- 

 lium constitutes the plant body proper, and absorbs the food 

 material from the substratum, which it ramifies, often causing 

 decay. The mycelium is frequently not visible, and the presence 

 of the fungus is not recognized until the so-called fruit bodies are 

 developed, as sometimes seen in the case of moldy oranges, 

 mildewed linen, and as illustrated by the common mushroom. 

 The mycelium has a cellulose wall which in some cases is modi- 

 fied to chitin, a nitrogenous substance related to animal cellulose 

 and found in crabs and other lower animals. The protoplasm 

 either occurs in a more or less delicate form lining the hyphae and 

 enclosing large vacuoles, or is comparatively dense enclosing 

 numerous small vacuoles. Many Fungi contain color substances 

 which are dissolved in the cell-sap and are of a quite brilliant hue. 

 One of the most interesting classes of substances produced by 

 Fungi is that of the ferments, including the oxidizing ferment 

 allied to laccase. They contain also amido-substances related to 

 lecithin ; fats ; carbohydrates, as trehalose and mannitol ; organic 

 acids, as oxalic, tartaric, malic, etc. ; and calcium oxalate may 

 be present in some cases. 



Reproduction in the Fungi is chiefly by means of asexual 

 spores, which arise in two ways. In the one case they are devel- 

 oped in a special cell or sporangium at the end of a mycelial thread 

 and are known as ENDOSPORES. In the other case they arise on 

 special hyphae, or directly from the mycelium, and are known as 

 EXOSPORES or conidia. There are also several modifications of 

 these two types of spores, which may be referred to later. 



Groups of Fungi. There are four principal groups of Fungi : 



1. Phycomycetes. 



2. Basidiomycetes. 



3. Ascomycetes. 



4. Fungi Imperfecti. 



The Phycomycetes, or Algal-like Fungi, are so called because 

 they show a certain relation to the Algae. 



The Ascomycetes are distinguished by having a sporangium of 



