34 INTRODUCTION. 



suggestion, but strictly analogous with its presence in the 

 Conjugatae, and indeed in plants in general, for it is well 

 known that starch is often most abundantly produced in the 

 seed. 



Of all the facts which indicate the vegetable nature of the 

 Desmidicce, this is undoubtedly the most important, since it 

 is the most easily subjected to the test of experiment. The 

 swarming of the zoospores is seen only now and then, and 

 it is not always easy to trace the process of coupling ; but 

 every person can apply the test for starch, and needs only 

 to bear in mind that unless granular matter be present there 

 is no starch*. 



The conjugation of the fronds in this family supphes an 

 equally striking fact in proof that it belongs to the vegetable 

 kingdom. I have already mentioned that we have amongst 

 the Algae many examples of the junction of two individuals!, 

 and the commingling and union of their entire contents into 

 a seed- like body for the purpose of reproduction. The case 

 of the Conjugatse is well known, and Mr. Hassall has proved 

 that the same phcenomenon takes place in other genera. 



* After the diluted tincture of iodine is applied, the free iodine should be 

 removed by the aid of heat, occasionally adding a little water to facilitate its^ 

 removal. This in great measure removes the brownish stain which obscures 

 the purple tint, and then on applying the highest powers of the microscope, 

 the peculiar colour of the iodide can in general be easily perceived. 



t So far as relates to the formation of the sporangium, each joint in the 

 Conjugatae must be considered a distinct individual. On the contrary, in the 

 Desmidiese, each frond resulting from division is merely a portion taken from 

 the original one, and like a graft or slip from an apple, a willow, or a car- 

 nation, retains every accidental peculiarity of the variety from which it 

 parted. Thus one pool may abound with individuals of Staurastrum de- 

 jectum or Arthrodesmus incus, having the mucro curved outwards ; in a neigh- 

 bouring pool every specimen may have it curved inwards ; and in another it 

 may be straight. The cause of the similarity in each pool, no doubt is, that 

 its plants are offsets from a few primary fronds. The above fact, I must 

 further remark, often renders it very difficult to decide whether a particular 

 form be really a distinct species or merely a variety, since its abundance will 

 not suffice to establish its claim to the rank of a species. 



