MARCH AX TI A POLYMORPHA. 8 1 



thallus. These two do not separate, and, indeed, were it 

 not for the two double rows of leaves along the anterior 

 (under) surface, which give rise to the two grooves with 

 their strengthening hairs, it would be difficult to show 

 that any branching had occurred. 



The pedicel of the female head is made up of exten- 

 sions of the tissues of the thallus, but without the 

 development of the wings. The head is formed by 

 sudden branching, and as dichotomous branching must 

 always be in pairs, it results in an even number of 

 branches which are spread out like a very widely open 

 fan. But counting the rays of the head always gives 

 an odd number, which is explained by the fact that the 

 growing point is not at the tip of the rays but at the 

 sinus between them, while the rays are formed, as in 

 the thallus, by the extension of tissue on either side 

 the growing point. Thus each ray, with the exception 

 of the ones nearest the cleft of the head, stands between 

 two growing points, while those next the cleft have a 

 growing point only on one side of them. The hairs of 

 the rays correspond with the hairs of the wings, and 

 extend into the grooves of the pedicel. 



If now we turn to the male branch, we shall 

 find the pedicel only differs from that of the female in 

 possessing no chlorophyll tissue on its posterior (upper) 

 surface. The tissues of the upper surface of the head 

 were at an early period of growth continuous with those 

 of the thallus, but, owing to some unknown cause, they 

 have not continued to expand along with those of the 

 ventral side in forming the pedicel. The head is made 

 up of branches, as in the female head, and like that is 

 not a radial structure, but zygomorphic. The cleft is 



