THALLOPHYTA. 643 



root-cell sends out a tube which grows into ;i young vegetative plant : if left in the 

 soil, the latter remaining damp, each root-cell germinates to form a peculiar thick- 

 walled plant called a hypiiosporaii'iiuiii which can withstand desiccation, and which 

 in water gives rise to a number of zoospores. 



The above facts may be briefly expressed by saying that any stage in the life- 

 history of Botr;/ili a in tends (1) on damp soil to form vegetative plants, (2) in water 

 to form zoospores or gametes, (3) in dry conditions to form resting cells. There 

 can, then, be no doubt that here, as in the case of Hydrodictyon, we are justified in 

 saying that the form which any given set of conditions tends to produce is adapted 

 to meet those conditions. 



Phyllosiphonacece. — Phyllosiphon Arisari is an Alga living in the intercellular 

 spaces of the leaves of Arisarum vulgare in Southern France and Italy. Its 

 thallus consists of a much-branched tube, the parietal protoplasm containing many 

 nuclei and small disc-shaped chromatophores. Unlike the Endosphairece, the pre- 

 sence of Phyllosiphon has a considerable effect on its host, causing blotches of dead 

 tissue to appear in the leaves. As the Alga can presumably assimilate quite well 

 for itself, this destruction of tissue is probably caused by withdrawal of water from 

 the cells of the host. The only kind of reproductive cells which Phyllosiphon is 

 known to produce are aplanospores (non-motile spores). These are formed by the 

 division of the whole of the protoplasm of the thallus. They are extruded by the 

 swelling of the inner membrane of the tube which bursts the outer membrane just 

 under a stoma, shooting out a jet of mucilage in which the aplanospores are em- 

 bedded. The latter germinate directly, the germ-tubes entering the leaf between 

 two epidermal cells. 



Vaucheriaceoz. — This family includes only the well-known and widely distri- 

 buted genus Vaucheria. Different species of Vaucheria grow in brackish and fresh 

 water, both running and stagnant, or in the air in damp situations. The thallus 

 consists of relatively coarse branched tubes, quite visible to the naked eye. The 

 interior of the tube is lined by a layer of protoplasm containing numerous disc- 

 shaped chlorophyll-grains and many nuclei. The Vaucheria-plant is fixed to 

 its substratum by short-branched, colourless processes, but, except in connection 

 with the formation of reproductive cells, transverse septa are not formed in the 

 tubes. 



The gametes of Vaucheria are formed in special organs, known as antheridia 

 and oogonia. The distinction of sex is very strongly marked, the male gametes or 

 spermatozoids being very small oval cells, each with two laterally inserted flagella, 

 while the female gametes or eggs are very large and quite motionless. The 

 antheridia are often spirally curved branches of the main tube, a transverse wall 

 separating the upper part of the spiral, the antheridium proper, from the lower part, 

 which is continuous with the cavity of the vegetative tube. Sometimes, however, the 

 antheridium is straight and club-shaped, and in other cases it may be separated 

 from the main tube by an intermediate cell. The thirty-five species of Vomcheria 

 are classified according to the characters of their antheridia. The oogonia ate 



