THE DICOTYLEDONES 



1773 



The stems are short and subterranean and are truly polystehc, each vascular 

 strand being surrounded by its own endodermis. In G. majucata pockets 

 in the stem cortex contain the Blue-green Alga, Nostoc gunnerae. It is 

 frequently cultivated in large gardens as a lakeside plant. The flowers are 

 produced on large fleshy axes and are individually inconspicuous (Fig. 

 1 661). The two petals are but slightly larger than the two minute sepals. 

 The two stamens are free, with short filaments, while the ovary is composed 

 of two carpels but is unilocular and contains one ovule. The embryo is tiny 

 and is buried in the endosperm at the upper end of the seed. This general 

 reduction of parts as compared with the other members of the family is 

 further seen in the embryo sac, which only contains four nuclei. The 

 genus Hippuris, though separated into a monotypic family by Engler, is 



now usually included here. H. vulgaris, 

 the Mare's-tail, is widely distributed 

 through i\rctic and temperate regions and 

 is also found in Antarctic America. 



Fig. 1661. — Gunnera manicata. Fig. 1662. — Callitriche obttisangiila. A, Flowering 



Inflorescence, which comes up shoot with terminal rosette of leaves. B, Male 



directly from the underground flower with two bracteoles and one stamen, 



stem. C, Female flower with two bracteoles and two 



united carpels. 



The Callitrichaceae, sometimes included in the Haloragidaceae, 

 comprises the single genus Callitriche (Fig. 1662) which contains small 

 herbaceous plants mostly found submerged in fresh water all over the 

 world. Some consider that only one or two species exist which are divisible 

 into nearly 100 varieties. Others elevate these varieties to species rank. We 

 have already referred to the group in connection with the Centrospermae 

 to which they may be related. Others such as Engler and Warming would 

 relate them to the Euphorbiaceae. They have also been included in the 

 Labiatae on account of their fruit of four nutlets. 



