88 A TEXTBOOK OF THEORETICAL BOTANY 



inside which is a peripheral layer of cytoplasm. The central part of the cell 

 is occupied by a large vacuole containing cell sap. The nucleus is suspended 

 in the centre of the vacuole by cytoplasmic strands. The most conspicuous 

 structure in the cell is the chloroplast, which consists of one or more spiral 

 bands lying in the peripheral layer of cytoplasm. It is the appearance of these 

 bands that has given the plant its name. Embedded along the chloroplast 

 are numerous pyrenoids around which is stored up the starch acquired during 

 photosynthesis. In section the chloroplast is usually T-shaped, the inner 

 flange being connected by fine filaments of cytoplasm to the nucleus. 



The filaments grow by simple cell division, each cell dividing by a wall 

 transverse to its length, and any cell of the filament being capable of division. 

 Such a type of growth, where any cell of the filament can divide, is spoken 

 of as intercalary growth, and may be contrasted with the apical growth 

 of higher plants, where division is restricted chiefly to the cells of the 

 growing point. 



Cell division in Spirogyra begins before nuclear division, not after it as 

 in higher plants, and the two processes are completed simultaneously. The 

 new transverse wall starts as a ring on the inner surface of the longitudinal 

 walls and grows inwards towards the centre. The cell does not necessarily 

 divide into two equal parts, for the new- wall lies exactly half-way between 

 the nuclei, wherever that may happen to be in the cell. The position of the 

 new wall is therefore fixed by the position of the nuclei and this position 

 is determined while the mother nucleus is still in the metabolic condition. 

 A similar condition exists in Cladophora. 



Nuclear division in Spirogyra is also exceptional in that the nuclear 

 membrane persists and divides with the nucleus, the spindle being formed 

 inside the nucleus instead of in the cytoplasm. 



Sexual Reproduction 



Spirogyra grows in dense masses, and in this condition the filaments at 

 the time of sexual reproduction come to lie side by side, probably as a result 

 of surface tension changes. In this position sexual reproduction is possible 

 by a process known as conjugation (Fig. 69). Most frequently this process 

 takes place in the spring, but it can be induced by the presence of a liberal 

 supply of sugar, or by a deficiency of nitrogen. The filaments adhere closely 

 to one another in pairs surrounded by a common investment of mucilage, 

 and then, preliminary to conjugation, opposite cells in adjacent filaments 

 develop short protuberances from their side walls, which are in contact with 

 each other from the start, and as they grow push the filaments apart. Each 

 pair of protuberances when united forms a conjugation canal. The wall 

 between them partially breaks down and a small conjugation pore is formed, 

 through which the protoplasts of the two opposite cells come into contact. 

 Conjugation by this method is called the ladder type from the appearance 

 of the two parallel filaments linked by the conjugation tubes. At this stage 

 the mucous sheath of the filaments disappears, 



