RECENT ADVANCES IN SCIENCE 551 



duration of the active period of the birch is stated to be 

 uniform at the tree Hmit, and the factors influencing this and 

 the minimal summer temperature are considered to determine 

 the altitude of this zone. Many of the birch woods are heathy 

 in character, with Vaccinium myrtillns, Deschampsia flexuosa, 

 Solidago-virgaurea, etc., and mosses, especially Dicranum. spp., 

 are often abundant, their prevalence being associated with a 

 poverty of lichens. Empetrum nigrum replaces Vaccinium 

 where the snow melts earlier, as around the birch trunks, or 

 even throughout the wood in situations which are very dry and 

 exposed, with thin humus. These woods occur in the subalpine 

 region with a vegetative period of about three months (mid-May 

 — August). In the alpine zone, this lasts only about 2\ months, 

 and the vegetation is mainly scrub, dominated by willows and 

 Betula nana, meadows on thin soil consisting of either tall 

 herbs, such as Geranium, sylvaticum, Mulgedium. alpinum, 

 Angelica, etc., or low-growing species, including Dry as, Sibbaldia, 

 Viola biflora, etc. Heaths also occur in which Empetrum, 

 Vaccinimn, Bryanthus, Diapensia, Loisleuria, etc., form growths 

 comparable to our Vacciniimi-Calluna heaths, or they are 

 dominated by grasses, amongst which Anthoxanthum and 

 Nardus stricta are prominent. The moorlands are chiefly 

 Carex moors or Eriophorum moors. Numerous subtypes are 

 described, and statistical data show the comparative areas 

 occupied by each. 



Miss Lyle, writing of the marine algas of Guernsey {Jour. Bot.), 

 recognises three zones, viz. the upper littoral, mid-littoral, and 

 sublittoral, in which the predominating species vary according 

 to the exposure. Thus, in the uppermost zone, species of 

 Cyanophyceae are associated with exposed conditions and 

 Pelvetia with moderate shelter. This latter condition also 

 favours Enteromorpha and Cladophora rupestris, whilst Porphyra 

 is associated with extreme shelter. In the second zone 

 Fuciis spp. dominate with F. vesiculosus and F. serratus in 

 shelter, and F. vesiculosus v. evesiculosus where exposed. In 

 the sublittoral zone, Laminaria digitata, Saccorhiza polyschides, 

 and L. saccharina are a series indicative of increasing shelter. 

 On the more exposed rocks Corallina squamata and Lithotham- 

 nion polymorphum form an undergrowth to the larger species. 

 The entire algal flora comprises 428 species, of which 53 spp. 

 are Northern and 20 Arctic ; 382 species are common to Guernsey 

 and the South of England, and 356 are also met with on the coasts 

 of Northern and Western France, showing a greater British than 

 continental affinity. 



Anatomy and Cytology. — Some years ago Bailey and Tupper 

 examined the lengths of vessels and tracheids in a number of 

 vascular plants, and found that the average for the tracheids 



