172 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



tacular requires the presence of spermatozoids, which are so 

 abundantly produced in many genera. The British Algse of 

 Dr. Harvey, the plates of the SupiDlement to English Botany, 

 the treatises of Derbes and Solier, together with that of 

 Thuret, so often quoted, exhibit numerous cases.* It is very 

 true that their necessity has not been proved by experiment, 

 but the bodies in question are in essential structure and outward 

 appearance, except perhaps the filiform appendages, similar to 

 those so common in Chlorosperras, which we know to be often 

 reproductive, besides which there are very numerous genera in 

 which their presence has not been verified. But even granting 

 the supposition that in many genera they do not exist, this is 

 no argument against the cases in which they do ; and as their 

 fecundative powers have been ascertained in Melanosperms, we 

 have no reason to doubt that they are similar in Rhodosperms, 

 where they are never known to germinate. 



147. Though the divisional and generic characters depend, 

 in great measure, on the nature and disposition of the fruit, it 

 will be impossible to separate these Algae into natural groups, 

 without paying attention to the structure of the frond. This 

 was first adopted by the younger Agardh, who, in consequence, 

 made many improvements in the disposition of species ; but, like 

 the venation of ferns, the characters so derived are sometimes 

 pushed to such an extent as to separate plants which really 

 ought to be united. Unfortunately the structure of these plants 

 is not always easily understood, especially without some practice, 

 in consequence of the quantity of mucilage which enters into 

 their composition, and of the transition from cells to filaments. 

 Much information on this head may be derived from Kutzing's 

 admirable plates, and the works of Dr. Harvey may be con- 

 sulted with great advantage. 



148. The structure of both Rhodosperms and Melanosperms 

 is often very difficult to make out, on account of the divi- 

 sions between the separate cells being extremely obscure. 



* Mrs. Griffitlis, to wliom we are indebted for so manj^ discov^eries, 

 had long observed these matters before their importance was recognised 

 by systematists, and I am indebted to her kindness for many illustrative 

 specimens. 



