186 SCIENCE PROGRESS. 



manufacture the substances which are requisite for the 

 development of the sagittate leaves. That the latter do 

 not occur in deep or rapid water is not due to the water, 

 but to the diminution of light. I succeeded in rearing 

 Sagittaria plants with leaves sixty cm. long and of ribbon 

 shape, and deep green colour, and to limit the plant to that 

 construction of leaf mainly by exposing it to less light. As 

 the Sagittaria is increased by means of tubers, one could 

 continue to rear them, doubtless, for generations in this 

 form, which differs so widely from the normal. In the same 

 manner moss-protonema, as such, can be cultivated for years; 

 moss plants only begin to appear on them when they are 

 exposed to light of a certain sufficient intensity. 



Among the Opuntias there are some species which 

 possess cylindrical shoots, others again in which they are 

 quite flattened ; the former are clearly the more primitive, 

 and if the more flattened stemmed Opuntias are exposed 

 to less intense light, they become cylindrical. In every 

 case the apex is cylindrical, but in many kinds it soon 

 gives rise to a flattened axis under the influence of light ; 

 but if the transforming agency is removed, the original 

 nature of the shoot becomes manifest. Similar results 

 were obtained with Muhlenbeckia platyclada and some 

 other plants. Other agencies can exert an influence analo- 

 gous to that effected by light. One of our most common 

 liver- worts, Frullania, has helmet-shaped pendants to its 

 leaves, the " auriculae," and they are organs which are able 

 to retain water by capillary attraction. If the plant is cul- 

 tivated under damp conditions, the auricle portion of the 

 leaves develops as ordinary leaf-lobes ; a similar function is 

 performed by the perforated, empty cells in the leaves of 

 the Sphagnum. Some Sphagnum species, however, only 

 possess ordinary chlorophy-containing cells like other mosses 

 when they grow in a submerged condition. On the leaves 

 of Poly trie hum there are on the mid-rib closely-packed 

 lamellae consisting of chlorophyll containing cells which re- 

 tain water in between them. It is possible to effect the 

 suppression of those lamellae also when the plant is culti- 

 vated in damp conditions. 



