Some Questions which they Suggest. 63 



formed in cells. It seems scarcely possible for organisms 

 living in the air to attain any considerable size or com- 

 plexity of form without the support of cell- walls, and 

 without the formation of vessels which assist the transfer 

 of nourishment from one part to the other. 



But with plants inhabiting the water a medium of nearly 

 the same specific gravity as the plant and drawing their 

 nourishment directly from this medium, the case is different, 

 and the possibility of such organisms attaining con- 

 siderable proportions and complexity of outward form is 

 shown by a considerable group of Alga, for which there 

 has recently been formed a class called Multinueleatce, 

 which includes four orders with considerable differences 

 amongst themselves, but which all agree in possessing no 

 cell-walls, and, under ordinary conditions, no septum 

 dividing one part from the other. Each organism is thus 

 a single protoplast. These unicellular organisms, as they 

 are often called, show a capacity for developing a vast 

 diversity of forms, many of them very beautiful, and many 

 of them strangely mimetic of the forms of higher plants 

 of the mosses, the lycopods, the conifers, the cactus tribe, 

 and the hymenomycetous fungi. Some of these organisms 

 reproduce sexually, others asexually ; some attain very 

 considerable size as in the genus Caulerpa, a beautiful 

 form of marine alga. "Nature," says Mr. Geo. Murray, 

 speaking of Caulerpa, " appears to have executed in the 

 form of this genus a tour de force in exhibiting the pos- 

 sibilities of the siphoneous thallus in showing that it ia 

 possible for a unicellular organism to display the varied 



