68 PART I. THE MORPHOLOGY OF PLANTS. [ 15. 



It is not possible to discriminate absolutely between a unicellular gemma and 

 a spore. There is, in fact, a gradual transition between this form of vegetative 

 multiplication and the simpler forms of spore-reproduction. 



In other planbs, multicellular gemmae are produced. They are 

 two-celled in Aneura multifida and some other Hepaticae ; whilst 

 in yet others the gemmae are multicellular flattened isobilateral 

 plates (Fig. 4) ; they may be developed in special receptacles 

 (cupules) on the upper surface of the thallus (Lunularia, Mar- 

 chantia), or on the margin of the leaves. In some Mosses flattened 

 gemmae are produced in receptacles formed of leaves at the apex of 

 the shoot, as in Tetraphis pellucida, and Aulacomnion androgynum : 

 and rounded tuberous gemmae are frequently formed on the 

 protonema. The prothallia of some Ferns (Trichomanes) are 

 reproduced by few-celled filamentous gemmae ; and that of Lyco- 

 podium PItlegmaria by ovoid tuberous gemmae. 



Among the Algae, Sphacelaria tribuloides, Melobesia call.itham- 

 nioides (Florideae), and Chara, produce multicellular gemmae. In 

 Sphacelaria and Melobesia, they are branches of the thallus : in 

 Chara they may be peculiar branches of the shoot (proembryonic 

 branches) or isolated subterranean nodes {starch- stars). 



In some Fungi (Ascomycetes and Basidiomycetes) compact 

 masses of tissue, which may be regarded as organs for vegetative 

 reproduction and are termed sclerotia, are formed on the mycelium. 

 In the Lichens, rudimentary branches of the thallus, termed 

 soredia, are produced and set free. 



In the Vascular Cryptogams and Phanerogams, vegetative re- 

 production of the sporophyte is generally effected by buds, the 

 leaves or stem of which have become swollen, serving as deposi- 

 tories for nutrient substances. These buds may be subterranean, 

 and then they are of considerable size, when they are termed bulbs 

 or corms according to their structure (see p. 40) : or the buds may 

 be borne on a swollen subterranean stem {e.g. potato-tuber) ; or 

 be associated with tuberous roots (e.g. many terrestrial Orchids). 

 Sometimes they are aerial, being borne on the stem ; on account 

 of their small size they are termed bulbils (e.g. Lilium bulbiferuw, 

 Dentaria, Nephrolepis tuberosa, etc.). In Psilotum, however, vege- 

 tative propagation is effected by small flattened gemmae, oval in 

 shape, and consisting of a few large cells forming a single layer. 



2. Spore-Reproduction. The term spore is applied to a specialised 

 asexual reproductive cell which' is capable, by itself, of giving rise 

 to a new organism. 



