K.— BOTANYj 189 



their annual cycle but in different habitats. The former is in a sense 

 more important than the latter, since habitat-forms may be expected to 

 represent distinct entities that in their respective environments will often 

 maintain constant differences. Brand's study of the Cladophoras 31 has, 

 however, shown how much an algal species can vary in the different seasons 

 of the year, and analogous studies of species of genera like Ulothrix, Stigeo- 

 clonium, Trentepohlia, &c, are urgently required. They will not only 

 lead to some solution of the ' species ' difficulty, but may as already 

 indicated afford valuable data in connection with possible points of con- 

 tact with higher plants. Moreover, they are readily compassed during 

 periodicity studies, such as have already afforded much information as 

 to the conditions determining appearance, abundance, and reproduction 

 of diverse Algae. 



An attempt has been made in various quarters to find a solution of the 

 species problem in the method of ' pure culture.' It is well, however, 

 to realise the limitations of the method, and it may be doubted whether, 

 except from the standpoint of physiology, the large amount of labour 

 that has been expended on such work has been justified. Two mono- 

 graphs of the genus Scenedesmus based on pure cultures have appeared 

 in recent years, but the authors do not agree in any way as regards the limits 

 of the species. It cannot be denied that, as a supplement to frequent 

 direct observation, pure cultures may be of considerable value and that 

 under certain circumstances (e.g. in the study of subterranean soil-Algae 

 or of endophytes) they are essential. The conditions are, however, 

 for the majority of Algae so artificial and of necessity in certain respects 

 so uniform as compared with those in nature, that the results obtained 

 require to be interpreted with great caution. 



Indoor conditions, especially in laboratories, are already a3 a general 

 rule sufficiently harmful to freshwater Algae, and the occurrence of various 

 bizarre forms in agar-agar or gelatine cultures, while of some interest 

 from the point of view of comparative morphology, cannot be regarded 

 as proving that such forms belong to the normal cycle in nature of the 

 Alga in question. Conversely because, in a pure culture, a given Alga 

 varies only between very narrow limits, that is no proof that these repre- 

 sent the full range of its morphological variation, but merely that under 

 the peculiar circumstances of a pure culture it exhibits this' habitat-form.' 

 No systematist would be satisfied with a knowledge of a higher plant 

 derived only from specimens grown in a botanic garden, where conditions 

 are by no means as artificial as they often are in an algal culture. 



I have offered these comments on the study of Algae in pure cultures, 

 not with any wish to deny the utility of the method in certain directions, 

 but in order to make clear that it can in no way replace direct observation 

 of the Alga in nature. Here alone we have the natural form, subjected 

 to the normal seasonal changes and other meteorological influences, 

 holding its own in competition with the other living members of its en- 

 vironment, and exposed to the influence of the frequent changes in the 

 organic and inorganic content of the water. It is a mistake to suppose 

 that careful periodic observations cannot in most cases lead to the desired 

 goal. The work will be laborious, but the labour will be repaid by the 

 31 Bot. CentraM. lxxix, 1899, p. 145. 



