DIV. I 



MORPHOLOGY 193 



pass into a resting condition (p. 305), in which they are more 

 resistant to injurious influences (desiccation, frost, heat). Such germs 

 are usually thick- walled, and only germinate on the return of 

 favourable conditions. 



C. Types of Reproductive Bodies. The germs which can develop 

 into plant bodies composed of many cells may themselves be 

 unicellular (spores) or multicellular (gemmae and seeds). Both kinds 

 may be produced irregularly on the plant, or be restricted to definite 

 regions, which are specially constructed for reproduction, and exhibit 

 great variety. These differences are of primary importance for the 

 division of plants into classes, orders, families, etc. Two types of 

 reproduction are ^ readily distinguished in plants of nearly all the 

 classes of the vegetable kingdom. 



In the first type, cells or multicellular bodies are formed which can 

 develop into a new independent individual on their separation 

 from the parent, either at once or after a period of rest. This 

 kind of reproduction is termed VEGETATIVE, ASEXUAL, or MONO- 

 GENETIC. 



In SEXUAL REPRODUCTION, the second of the two modes of 

 reproduction, two kinds of reproductive cells, each of which carries 

 the characters of the organism producing it, are formed, but 

 neither is directly capable of further development, and both perish 

 in a very short time, unless opportunity is given for their fusion with 

 each other. Not until the one cell has fused with the other cell does 

 the product acquire the capacity of development and growth. This 

 mode of reproduction is termed SEXUAL or DIGENETIC reproduction. 



Most plants have both methods of reproduction. Sexual repro- 

 duction is wanting only in the lowest groups (the Bacteria, Cyano- 

 phyceae, and some Algae and Fungi). Some plants have several 

 methods of asexual reproduction. 



In certain exceptional cases a sexual cell may proceed to develop 

 further without fertilisation. This is termed PARTHENOGENESIS ( 94 ). 

 This has been found in the vegetable kingdom in Cham crinita, one of the 

 Algae, and in the development of the embryo from the unfertilised ovum 

 in a number of families of higher plants (Compositae, Ranunculaceae, 

 Rosiflorae, Thymeleaceae, Urticaceae), and in the Marsiliaceae. 



The process of fertilisation of sexual cells may, in particular cases, be replaced 

 by the fusion of the nuclei of adjoining vegetative cells ( 95 ). This is the case in the 

 prothallium of certain cultivated forms of Ferns (e.g. of Dryopteris (Lastraea) and 

 Athyrium). The product of this fusion effects the reproduction, the sexual organs 

 of the prothallium being reduced. 



D. Alternation of Generations ( 6 ). In plants there is frequently 

 an alternation of two generations differing in their modes of repro- 

 duction ; these may be morphologically distinct and independent 

 individuals. The life-history of such a plant is thus composed of 







