52 MORPHOLOGICAL DEVELOPMENT . 



is just the character of the successive leaves in the higher 

 types. As shown in Fig. 47, each leaf is usually composed 

 of two unequal lobes. 



A natural concomitant of the mode of growth here de- 

 scribed, is, that the stem, while it increases longitudinally, 

 increases scarcely at all transversely : hence the name 

 Acrogens. Clearly the transverse development of a stem, is 

 the correlative, partly of its function as a channel of circula- 

 tion, and partly of its function as a mechanical support. 

 That an axis may lift its attached leaves into the air, implies 

 thickness and solidity proportionate to the mass of such 

 leaves ; and an increase of its sap-vessels, also proportionate 

 to the mass of such leaves, is necessitated when the roots 

 are all at one end and the leaves at the other. But in the 

 generality of Acrogens, these conditions, under which arises 

 the necessity for transverse » growth of the axis, are absent, 

 wholly or in great part. The stem habitually creeps beloT/ 

 the surface, or lies prone upon the surface ; and where it 

 grows in a vertical or inclined direction, does this by at- 

 taching itself to a vertical or inclined object. Moreover, 

 throwing out rootlets, as it mostly does, at intervals through- 

 out its length, it is not called upon in any considerable de- 

 gree, to transfer nutritive materials from one of its ends to 

 the other. Hence this peculiarity which gives their name 

 to the Acrogens, is a natural accompaniment of the low 

 degree of specialization reached in them. And that it is an 

 incidental and not a necessary peculiarity, is demonstrated 

 by two converse facts. On the one hand, in those higher 

 Acrogens which, like the tree-ferns, lift large masses of 

 foliage into the air, there is just as decided a transverse ex- 

 pansion of the axis as in Exogens. On the other hand, in 

 those Exogens which, like the common Dodder, gain sup- 

 port and nutriment from the surfaces over which they creep, 

 there is no more lateral expansion of the axis than is habit- 

 ual among Acrogens. Concluding, as we are thus fully justi- 

 fied in doing, that the lateral expansion accompanying longi- 



