^42 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



and planogametes, or spermatozoids and eggs: in very many forms no reproductive 

 cells are known. The higher forms of Siphoneae often produce plant-bodies of very 

 definite and characteristic external form, and of considerable size. In some cases 

 these simulate the external form of various higher plants. 



Families: BotrydiacecB, Phyllosiphonacece, VaucheriacecB, Bryopsidacece, 

 CaulerpacecB, Godiacece, Valoniacece, Verticellatce. 



Botrydiacece. — Botrydium granulatum is a little plant found growing espe- 

 cially on loam at the damp edges of ponds and ditches. It consists of a club-shaped 

 or balloon-shaped green shoot-portion, about 1-4 millimetres in diameter, continuous 

 with a simple or branched tubular colourless root-portion which is embedded in the 

 substratum. The entire plant consists of a single cell, that is to say, its cavity is 

 •continuous throughout. The wall is lined with a thin layer of protoplasni, which 

 contains many nuclei, and, in the shoot, a net-like chlorophyll-layer. 



BotrydivMi can reproduce itself in very various ways, according to the incidence 



of external conditions. The simplest form of propagation is by budding, which 



takes place under conditions favourable to the ordinary vegetation of the plant. 



The shoot-portion of a small vegetative plant sends out a process which swells to 



the size of the mother shoot, puts out a colourless root, and is then constricted off 



to form a separate plant. But if the plants are covered with water they cannot 



go on growing comfortably, and accordingly the protoplasm breaks up to form a 



number of zoospores, each with a single flagellum and two lateral chromatophores. 



The mass of zoospores is subjected to considerable pressure by the swelling up of a 



ring-like area of the wall, and the tension becomes so great as to rupture the wall 



in the centre of the ring and expel the mass of zoospores into the water. On damp 



soil the zoospores come to rest, and germinate to form new plants. If a zoospore 



cannot escape from the water it enters on a resting stage, which gives rise to a new 



plant directly it finds itself on damp soil. Further, if a young plant is exposed to 



bright sunlight, its protoplasm breaks up into a number of spherical cells, each of 



which puts on a cell-wall. If now these spherical cells (gametangia) are placed in 



water, the contents of each breaks up into spindle-shaped, biflagellate gametes, 



which conjugate in pairs to form zygotes. These zygotes can rest for a longer or 



shorter time, but if placed on damp earth they at once germinate to form new 



plants. If, on the other hand, the gametangia are placed in water after being kept 



for two years they give- rise to biflagellate cells which rather resemble the gametes, 



but which on damp soil germinate directly to form vegetative plants. Finally, if 



the gametangia are at once placed on damp soil, their contents does not break up, 



but the whole gametangium germinates and produces a plant. Supposing a large 



Botrydium-plant, with a balloon-shaped shoot, be exposed to insolation (bright 



light) its contents, instead of forming gametangia, travels down into the root, and 



the protoplasm there divides to form rows of root-cells, each with an independent 



cell-wall. If a root containing root-cells be placed in water, the protoplasm of each 



cell breaks up into a number of zoospores: if the root be placed on damp earth each 



