600 LECTURE XXII. 



is propagated by means of tubers which are modified sub- 

 terranean shoots bearing numerous buds ; and the Dahlia, 

 by means of its tuberous roots which give rise on germination 

 to shoots. 



Although vegetative reproduction is in many cases very 

 effectual, yet all plants possess the property of reproducing 

 themselves by means of spores. 



A spore is a single cell, a statement to which the " multi- 

 cellular" or better the " compound" spores of some Fungi are 

 not real exceptions, for these are, in fact, spore-aggregates. 

 It consists, in the majority of cases, of a nucleated mass of 

 protoplasm, enclosing starch or oil as reserve nutritive material, 

 surrounded by a cell-wall. In those cases in which the spore 

 is capable of germination immediately on the completion of 

 its development, the cell-wall is a single delicate membrane 

 consisting of cellulose : but in those cases in which the spore 

 may have to, or must necessarily, pass through a period of 

 quiescence before germination, the wall is thick and may con- 

 sist of two layers, an inner, the endospore, which is delicate 

 and consists of cellulose, an outer, the exospore, which is thick 

 and rigid, frequently dark-coloured and beset externally with 

 spines or bosses, and which consists of cutin. In some plants, 

 particularly among the Algae and also in some Fungi (Pero- 

 nosporeae, Saprolegnieae, Myxomycetes, Chytridiaceae), spores 

 are produced which are for a time destitute of any cell-wall, 

 and are further peculiar in that they are motile, on which 

 account they are termed zoospores. They move, sometimes 

 in an amceboid manner by the protrusion of pseudopodia, but 

 more frequently by means of cilia (see p. i). The zoospore 

 eventually comes to rest, surrounds itself with a cell-wall, 

 and then germinates. In any case, a spore is a cell which 

 is capable, by itself, of giving rise to a new individual on 

 germination. 



The main point concerning the development of spores is 

 that they are produced in one of two ways, either asexually 

 or sexually. In the former case they are directly produced 

 from the protoplasm of a single reproductive organ, to which 

 we may apply the general term Sporangium ; hence we speak 



