96 PLANT MORPHOLOGY 



cells for reproduction. Differentiation becomes marked among the 

 brown algae, especially in the Laminariales and Fucales, where the body- 

 consists of distinct vegetative organs within which simple tissues may be 

 formed. A highly differentiated vegetative body is also characteristic of 

 the Charophyceae and many marine Siphonocladiales and Siphonales. 



Asexual Reproduction. In reproduction, as in vegetative structure, 

 the algae show a progressive advance. Most unicellular forms increase 

 in number by fission, which is merely reproduction by cell division and is 

 obviously the most primitive method in the plant kingdom. Among 

 multicellular forms cell division results in growth and, to make repro- 

 duction possible by other means than fragmentation, cells must be Ub- 

 erated from the parent. The spores of algae are merely detached cells 

 with the capacity of directly producing a new plant. They result not 

 only in a multiplication of individuals but in their widespread distribu- 

 tion. Spores may be formed from a cell with or without previous division 

 of its protoplast. The commonest kind of spores in the algae are zoo- 

 spores — naked cells ' with cilia. Nonmotile spores with a cell wall 

 (aplanospores and akinetes) are generally formed in response to adverse 

 environmental conditions, to which they are very resistant. Obviously 

 they have been derived from zoospores that have lost the power of loco- 

 motion. The same may be true of the nonmotile spores of the red algae. 

 Fission and spores produced by a haploid plant body are a means of 

 accompHshing vegetative or asexual reproduction because no reduction 

 of chromosomes is involved. This is the only kind present in the blue- 

 green algae, flagellates, dinoflagellates, many diatoms, and a few green 



algae. 



Like vegetative spores, the spores produced by two successive divisions 

 of a diploid cell, involving a reduction of chromosomes, are usually 

 regarded as asexual. In reality, however, they belong to the sexual life 

 cycle, since meiosis is always a necessary conseciuence of a previous 

 gametic union. Although such spores are functionally equivalent to the 

 zoospores and aplanospores produced by a haploid plant body, they are 

 not homologous with them, and should be designated as meiospores. 

 Meiospores are produced by the zygote in such green algae as Ulothrix 

 and Oedogonium, and by the sporophyte in all algae with an alternation 

 of generations. 



In most green algae any ordinary vegetative cell is capable of producing 

 spores. In nearly all the brown and red algae, however, spores are not 

 borne in transformed vegetative cells but in sporangia, which are cells 

 specialized for reproduction. Sporangia differ from ordinary vegetative 

 cells in size or shape and sometimes are restricted to definite parts of the 



body. 



Sexual Reproduction. Sexual reproduction is accomplished by 

 gametes and represents a distinct advance over reproduction by vegeta- 



