Adaptation in the Tristichacecs and Podostemacece. 537 



family, though only, many of them, slightly modified from " common " types, 

 occupy the same positions as the highly modified members of the Podostemacese 

 proper, whether in swiftly moving or in slowly moving water. Now if such 

 simple forms as these suffice to occupy the positions, there can be no adapta- 

 tional need for the complex forms of the Podostemacese proper. But to return 

 to the structure : the most " primitive " - looking species is undoubtedly 

 Tristicha ramosissima Willis, which grows in South-West India. The primary 

 axis of this plant is not known, but as the creeping roots are just like those 

 of other forms, it may be taken for granted that it gives rise to creeping 

 closely attached roots, running over the rocks in all directions. Upon these 

 are borne endogenous " secondary " shoots, which in this plant grow out in a 

 fairly normal way to a length of a foot or so. They are not dorsi ventral, and 

 bear the flowers, which also are not dorsiventral, but have the formula 

 P3, A3, G(3), and are perfectly regular. A feature of the plant is the numerous 

 "ramuli" or shoots of limited growth, which here are in no sense dorsi- 

 ventrally arranged or constructed. Now in this plant the only adaptations, 

 if adaptations they can be called, to the mode of life, are (1) the great and 

 regular development of secondary shoots on creeping roots, and (2) of haptera 

 or organs for clinging to the rock, and (3) the absence of intercellular spaces. 

 But I have no hesitation in affirming that this plant could live quite com- 

 fortably in any of the localities occupied by the members of the two families, 

 with the exception of those in which during the growing season the water was 

 liable to be less than an inch or an inch and a half in depth. Life in 

 shallower water is quite easy for another species of Tristicha — hypaoid.es. 

 But in this one begins to see increasing dorsiventrality in the plant. Once 

 placed upon a rock, in which downward growth is impossible, and dorsi- 

 ventral or plagiotropic direction of growth ensured by the creeping roots, 

 the impetus to dorsiventrality appears to be given, and the whole further 

 evolution of both families seems to be chiefly in the direction of increasing 

 dorsiventrality, both in the vegetative and the floral organs. In T. hypnoides 

 the main secondary shoot begins to show some dorsiventrality and the ramuli 

 have become very distinctly tristichous, with one upper and two more lateral 

 rows of leaves. The flower is also dorsiventral in having lost the upper and 

 one lower stamen. In a new species of Tristicha which has lately been 

 discovered near to Rio by Snr. F. Tamandare de Toledo, the ramuli are 

 not only tristichous, but lie down flat upon the rock, and have the upper 

 row of leaves smaller than the laterals, while the secondary shoots have 

 become flat expanded " stem-thalli." Leaves are borne on the upper 

 surface of the flat expansion (which lies flat upon, and is attached to, the 

 rock), and ramuli as well as flowers on the margins. The flowers have 



