

Ch. X] THE THALLUS PLANTS 397 



Spores, indeed, are the characteristic reproductive bodies of 

 Thallophytes, though they are not confined to that group, 

 but are shared with Bryophytes and Pteridophytes, and 

 can even be traced morphologically in Spermatophytes. A 

 spore is typically a one-celled body capable of growth to a 

 new plant. The purely asexual spores are either ciliate or 

 flagellate, free-swimming, zoospores (swarm spores), as 

 developed from vegetative cells of many Algae and some 

 Fungi (Fig. 213), or else they are smooth round bodies formed 

 in sporangia whence they are scattered by wind, as in 

 Molds (Fig. 214), or they are abstricted in the air from naked 

 hyphae, as in the conidia of many Fungi (Fig. 319), or they 

 are formed in groups of four, as in the tetraspores of the 

 Red Algae (Fig. 311); and other special kinds occur in 

 particular groups of Algae and Fungi. Other spores, while 

 themselves asexual, are formed in a regular alternation with 

 sexual reproduction, as in case of the carpospores of the 

 Red Algae, and the ascospores and (originally) the basidio- 

 spores of the higher Fungi. The word " spore" is also 

 applied to the fertilized cells resulting from sexual fertiliz- 

 ation, — zygospore when the uniting cells are alike, and 

 oospore when unlike. The term resting spore has not a 

 morphological but a physiological connotation, describing 

 any spore which by thick wall and suspended vitality can 

 survive a lengthened period unfavorable for active life. The 

 sexual reproduction of Thallophytes is effected through the 

 union of sex cells, or gametes (page 303). These are either 

 alike, when they are called isogametes, and the resultant 

 fertilized cell is a zygospore or zygote, or else they are 

 unlike, heterogametes, consisting usually of a smaller 

 sperm cell and a larger egg cell (page 304), the resultant 

 fertilized cell being called an oospore. In the lower forms 

 zygospores and oospores grow directly to new plants, but 

 in the higher kinds, both Algae and Fungi, they develop 

 into new structures, called cystocarps, ascocarps, etc., 

 and these produce many true spores, as will later be 



