APPENDIX 



449> 



timum, where the separate carpels are about 6 mm. long and about 

 4 mm. broad, there is a variety with longer and relatively narrower 

 fruits (9 or 10 mm. long, 4 mm. broad), which seems to be confined 

 to lava rocks at the coast, and does not like the usual form grow 

 also on the beach. 



The locality in which I found the strand flora most developed 

 was on the west side of the low sandy isthmus which connects La 

 Isleta with the north-east corner of Grand Canary. Euphorbia 

 par alias was the most frequent of the beach plants and extended 

 inland for some hundreds of yards over the dunes. Amongst other 

 plants growing on the sandy shore were Atriplex poiiulacoides, 

 Frankenia, Heliotropium, Mesembryanthemum crystallinum, M. 

 nodiflorum, Zygophyllum, etc. The last named also grew along the 

 upper drift line where the beach was shingly and on the rocks border- 

 ing the beach at La Isleta. 



A few remarks on the modes of dispersal of some of these plants 

 may here be made. We cannot exclude human agency in the case 

 of the two species of Mesembryanthemum, which were extensively 

 cultivated in the group in the eighteenth century for the extraction of 

 soda, according to Samler Brown's Guide to the Canary Islands (1905,. 

 d, 9). These two plants grow also on the Salvages (Lowe's Florulas' 

 Salvagicce Tentamen, 1869), and were employed in making barilla, 

 (soda) by men engaged in the trade, who often visited the islands for 

 that purpose (Ibid.). The dispersal of the carpels of Crithmum 

 maritimum across the sea by the currents and locally by the winds 

 is discussed in my book on Plant Dispersal (p. 542). It is there 

 remarked that they are very buoyant and can float for several 

 months in sea-water. They are so light that a strong wind blows 

 them along the beach and up the faces of the sea-cliffs. (More 

 recent experiments showed that the carpels in some cases germin- 

 ated after floating for thirteen months in sea-water.) 



I experimented on the fruits of a species of Zygophyllum that 

 nourished on the beach of the low isthmus connecting La Isleta 

 with the north-east corner of Grand Canary. It is probably Z. 

 fontanesii, a littoral plant found also in the Cape Verde Group (Schmidt 

 and Coutinho) and on the Atlantic coast of Morocco (Hooker, p. 339). 

 From the results of the experiments below given it may be inferred 

 that the species tested has but a limited capacity for dispersal by 

 currents. It might, however, accomplish a sea-passage of from 

 100 to 200 miles. This would be quite sufficient for its derivation 

 in the case of the Canary Islands from the neighbouring African 

 coast and for its distribution over that group. 



The dry cocci of this species of Zygophyllum are 7 to 8 mm. long. 

 They possess a solitary crustaceous seed (2-5 mm. in length) in the 

 midst of spongy air-bearing tissue, and are so light in weight that 

 like the carpels of Crithmum maritimum they are blown about on 

 the beach by the winds. They would have possessed similar great 

 floating powers, did they not tend to gape at the inner angle. As 

 it is, the sea-water soon penetrates through the opening, and in 

 consequence they float for only eight or ten days. The seed itself 

 sinks. 



