10 



ALG.E. 



differentiation, as in many unicellular Alga 3 , or in multicellular 

 Alga? of the lower order, which are then either equally developed 

 in all directions (e.g. Pleurococcuf, Fig. 47), or form flat cell-plates 

 (Merismopedium) or threads (Oscillaria, Fig. 21). The first step in 

 the way of differentiation appears as a difference between apex 

 and base (Sivularia, Porphyra) ; but the division of labour may 

 proceed so that differences may arise between vegetative and 

 reproductive cells (CEdogonium, Fig. 54) ; hairs and organs of 

 attachment (rhizoids and haptera), which biologically serve as 

 roots, are developed, and even leaves in certain forms of high 

 order, belonging to different classes (e.g. Caulerpa, Fig. 59 ; 

 Characeoe, Fig. 61 ; Sargassum, Fig. 72 ; and many Floridea?). 



The nonsexual reproduction takes place vegeiatively, in 

 many instances, simply by division into two, and more or less com- 

 plete separation of the divisional products (Diatomaceae, Desmi- 

 diaceoe (Fig. 36), many Fission-plants, etc.), or by detached portions 

 of the thallus (e.g. Caulerpa, Ulra lacttica, etc. ; among many Schizo- 

 phyceSB, small filaments known as hormogonia are set free), or 

 asexually by special reproductive cells (spores) set free from the 

 thallus ; these may be either stationary or motile. The stationary 

 reproductive cells (spores) may either be devoid of cell-wall (te- 

 traspores of the Florideie), or may possess a cell-wall ; in the latter 

 case they may be formed directly from the vegetative cells, gene- 

 rally by the thickening of the walls (akinetes), or only after a 

 process of re-juvenescence (aplanospores) . Aplanospores, as well 

 as akinetes, may either germinate immediately or may become 

 resting-cells, which germinate only after a period of rest. 



THE MOTILE ASEXUAL REPRODUCTIVE CELLS are spherical, egg- or 

 pear-shaped, naked, swarmspores (zoospores), which have arisen in 

 other cells (zoosporangia), and propel themselves through the 

 water by means of cilia ; or they are Phyto-Amceba?, which have no 

 cilia and ci'eep on a substratum by means of pseudopodia. The 

 cilia, which are formed from the protoplasm (in the Bacteria, 

 however, from the membi'ane), are mostly situated at the pointed 

 and colourless end, which is directed forwards when in motion, 

 and are 1, 2 (Fig. 5 B), 4 or more. Both the cilia in the Brown 

 Alga? are attached to one side (Fig. 65); they are occasionally 

 situated in a circle round the fi'ont end (GEdogonium, Fig. 6 a, 

 and Derbesia), or are very numerous and situated in pairs dis- 

 tributed over a large part or nearly the whole of the zoospore 

 (Vauclieria). Besides being provided with one or more nuclei 



