DIV. i MORPHOLOGY 195 



sometimes on the leaf-blade, e.g. in the notches of the leaf margin in Bryophyllum, 

 and on the leaves of Cardamine pratensis. The leaves of Begonia, Drosera, etc., 

 only produce buds after they have been separated from the plant. 



Many herbaceous perennials, without forming special organs of vegetative repro- 

 duction, increase in number of individuals by the decay of the older portions 

 of their branched rhizomes isolating the branches. Among Sea-weeds also the 

 mechanical action of the surf may separate portions of the thallus which can grow 

 into new tlialli. Caulerpa is propagated in this fashion. 



2. The Formation of Reproductive Cells 



(a) Asexual Reproductive Cells (Spores). Many unicellular 

 Thallophyta (Flagellata, Bacteria, Cyanophyceae, Diatomeae) are 

 multiplied vegetatively by dividing into two, the daughter cells 

 separating from one another. In others, such as the Protococcaceae, 

 the protoplast within its wall divides into several or many daughter 

 protoplasts ; these separate from one another and emerge from the 

 parent cell through a pore or split in the cell wall. The unicellular 

 organism in these cases has at the period of reproduction become 

 converted into a receptacle containing the germ cells, or a SPORAN- 

 GIUM ; the germ cells which give rise to daughter organisms may be 

 termed ENDOSPORES or SPORANGIAL SPORES. 



Among the multicellular Thallophytes simply-organised forms are 

 met with that might be regarded as cell colonies, in which the body 

 sometimes dissociates into the individual cells ; these then serve for 

 vegetative multiplication. In other forms the protoplasts of all the 

 cells, usually after preliminary division into daughter protoplasts, 

 emerge at the period of reproduction from the cells which have thus 

 become sporangia. 



The more highly organised multicellular Thallophyta exhibit a 

 division of labour, only some portions or cells, which often have a 

 definite structure and position on the thallus, producing asexual 

 reproductive cells. Such spore-producing parts are often united in 

 numbers to form fructifications of more complex structure. 



Thus in many Fungi the hyphae concerned in reproduction become associated 

 within the soil into more or less massive and variously-shaped FRUCTIFICATIONS 

 which later emerge to the surface. 



The mode of origin of the spores is similar in the higher and 

 lower forms. In many Fungi germ cells are isolated by budding and 

 constriction from certain hyphae as EXOSPORES or CONIDIOSPORES 

 (Fig. 224). In other Fungi and in the majority of the Algae the 

 asexual cells originate as ENDOSPORES or SPORANGIAL SPORES from 

 the protoplasts of certain cells (SPORANGIA), and emerge through 

 openings in the wall of this (Figs. 225, 231 sp). 



The asexual spores of the Thallophyta are in part adapted to 

 distribution by means of water, as in the case of many sporangial 

 spores of Algae and Fungi. These spores are naked, without a cell 



