340 A TEXTBOOK OF THEORETICAL BOTANY 



products of an associated species. Staling implies the depression or stoppage 

 of growth, and it is due to the accumulation of staling substances of an 

 excretory character in a limited quantity of growth medium, so that a toxic 

 concentration is reached. It seems that such substances may be of quite 

 simple character, and it has been shown that bicarbonates, and perhaps 

 ammonia, are the factors chiefly influential. 



The Origin of the Fungi 



There are two possible ways in which the Fungi may have originated, 

 and both receive some support in the light of modern knowledge. The 

 older view was that the whole of the Fungi were derived from the Algae by 

 loss of chlorophyll, and that the Fungi are nothing more than Algae which 

 have become specialized to heterotrophic nutrition. As a result of parasitism, 

 the rapid method of asexual reproduction has to a large extent replaced the 

 more elaborate sexual method. Sexual reproduction, though still clearly 

 seen in many of the less specialized types, has become progressively simplified 

 until either only vestiges of a sexual apparatus remain or they have even 

 completely disappeared. Supporters of this view point to the similarity of 

 many Phycomycetes to Green Algae and of Ascomycetes to the Rhodophyceae. 

 Even among the examples studied in this book we can see a distinct similarity 

 between Pythium and Vauchen'a, or between Miicor and Spirogyra. The 

 female reproductive organ of Aspergillus is very similar to the carpogonium 

 of Batradiospermiim. The ascogenous hyphae of Ascomycetes may be 

 compared to the gonimoblast filaments in the Rhodophyceae. Further, a 

 study of the fungal components of certain Lichens emphasizes the remarkable 

 similarity between the sexual reproductive organs of these Ascomycetes and 

 of the Rhodophyceae. 



It is not surprising, therefore, that the older mycologists considered the 

 algal origin of the Fungi extremely possible, in fact in early times the Fungi 

 were classified as non-chlorophyllous types following their supposed algal 

 counterparts. 



More recently, as a result of more careful study of the Fungi, this simple 

 view has become less and less satisfactory. It has been pointed out that very 

 few organisms exist at the present day which could be regarded as inter- 

 mediate between Algae and Fungi, such as might be expected to occur if 

 such a transition was still going on. PoIysipJumia fastigiata is known to be a 

 partial parasite on the Brown Wrack, AscophyUum nodosum, that is to say, it 

 grows more vigorously as a parasite than by itself, nevertheless it is still an 

 obvious Alga. Two other red algal genera, HarreyeUa and Choreocolax, 

 have gone further and have completely lost their chlorophyll ; they are still, 

 however, typical Red Algae in structure, and no one would suggest that they 

 resembled any known Fungus. Again the fungal cell wall is, with few 

 exceptions, composed of completely difl^erent substances from those found 

 in Algae. The storage of glycogen again is not an algal character, but is far 

 more characteristic of animals. Heterothallism, at any rate in the higher 



