562 MICKOSCOPIC FOKMS OF VEGETABLE LIFE THALLOPHYTES 



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Although many of the plants belonging to the family Siphonaceae 

 attain a considerable size, and resemble the higher seaweeds in their 

 general mode of growth, yet they retain a simplicity of structure so 

 extreme as to require them to be ranked among the simpler thallo- 



phytes. They are inhabitants 

 both of fresh water and of the 

 sea, and consist of very large 

 tubular cells, which often ex- 

 tend themselves into branches, 

 so as to form an arborescent 

 frond. These branches, how- 

 ever, are not separated from 

 the stem by any intervening 

 partition, except those parts 

 where the generative organs 

 are produced ; but the whole 

 frond is composed of a simple 

 continuous tube, the entire 

 contents of which may be 

 readily pressed out through an 

 orifice made by wounding any 

 part of the wall. The genus 

 Vaucheria may be selected as a 

 particularly good illustration of 

 this family, its history having 

 been pretty completely made 

 out. Most of its species are 

 inhabitants of fresh water, but 

 some are marine ; and they 

 commonly present themselves 

 in the form of cushion-like 

 masses, composed of irregularly 

 branching filaments, which, al- 

 though they remain distinct, 

 are densely tufted together and 

 variously interwoven. Some 

 species form dense green mats 

 on damp soil in flower-pots, &c. 

 The formation of motile gonids 

 or zoospores may be readily 

 observed in these plants, the 

 whole process usually occupying 

 but a very short time. The 

 extremity of one of the filaments 

 usually swells up in the form of 

 club, 



FIG. 426. Successive phases of generative 

 process in Vaucheria sessilis : at A are 

 seen one of the ' horns ' or antherids (a) 

 and one of the oogones (6), as yet un- 

 opened; at B the antherid is seen in 

 the act of emitting the antherozoids (c), 

 of which many enter the opening at the 

 apex of the oogone, whilst others (d) 

 which do not enter it display their cilia 

 until they become motionless ; at C the 

 orifice of the oogone is closed again by 

 the formation of a cellulose coat around 

 the ob'sphere, thus constituting an oospore. 



a club, and the endochrome 



accumulates in it so as to give it a darker hue than the rest ^ 

 a separation of this part from the remainder of the filament, 

 by the interposition of a transparent space, is next seen ; a new 

 envelope is then formed around the mass thus cut off; and at last 

 the membranous wall of the investing tube gives way, and the zoo- 



