THE ALG^E 161 



division succeeds the transverse one ; and if eight are to 

 be formed, another division again, at right angles to the 

 other two, completes the partition of the contents. 



In any case the resulting zoospores are pear-shaped, 

 contain each one nucleus, and have the chloroplast in the 

 broader part of the cell, while the pointed end is made 

 up of clear protoplasm. In each zoospore is a " pulsating 

 vacuole " which alternately contracts and expands about 

 every ten or fifteen seconds ; in this respect these vegetable 

 cells precisely resemble some of the unicellular animals. 

 At the point are the four long cilia by which the 

 locomotion is brought about. The mother cell-wall 

 breaks down at one side, and the zoospores escape. Their 

 movements are very active, and they have formed the 

 subject of some of the best observations on the locomotion 

 of vegetable cells. Eelatively to their own length their 

 speed is great. A zoospore will travel through twice or 

 three times its own length in a second, while the fastest 

 ship requires from ten to fifteen seconds to traverse its 

 own length ; on the other hand, zoospores are so small 

 that their actual speed is slow (about a metre in an 

 hour). 1 Considering the distance they are required to 

 travel, we may say that their locomotion is very active. 

 Under the microscope their size and speed are magnified 

 together, so that we are enabled to realise the true 

 relation between their movements and their dimensions. 



We saw some time back (pp. 70 and 136) that the 

 spermatozoids of the higher Cryptogams are sensitive to 

 certain chemical substances when dissolved in the water. 

 Zoospores of Algae are also sensitive, though in different 

 ways ; in this case they react especially to the influence 



1 Noll, iii Strasburger, Noll, Schenck, and Scliimper, Lehrluch der 

 Botanik. 

 11 



