154 DISTRIBUTION OF THE VEINS. 



uniting it with the midrib, and is in its natural state scarcely 

 connected with the surrounding parts, except by the cuticle which 

 envelopes the whole, the leaf will perform its functions nearly as 

 well when thus subdivided, as when entire. 



231. It is curious that such a separation should sometimes 

 take place under the influence of natural causes. The Banana 

 and Plantain of tropical climates, have leaves of this kind ; and 

 when they grow in situations, in which they are much exposed 

 to the wind, its action splits them up in this manner, from which 

 they do not appear to suffer. These plants are sometimes grown 

 in hot-houses in England ; and then, being completely sheltered, 

 the expansion of their leaves is preserved entire, which seldom 

 happens in their native localities. In whichever direction the 

 veins are arranged, the general character of the leaf is the same ; 

 and hence the leaves of Endogens are spoken of zsparallel- 

 veined, by which character they are distinguished, with but few 

 exceptions, from those of the Cryptogamia on one hand, and, as 

 will presently appear, from those of Exogens on the other. 



232. The form and mode of subdivision of the system of 

 veins in Exogens, are extremely irregular ; but there is a cha- 

 racter common to all, by which the leaves of this group may 

 be distinguished, without much difficulty, from those of the 



Fio. 63. EXOGENOUS LEAF. 



others. There is usually a midrib, or prolongation of the foot- 

 stalk along the centre of the leaf, from which the smaller veins 

 arise ; but sometimes the petiole subdivides at once into several 

 subordinate veins, which run from one extremity of the leaf to 

 the other, nearly parallel with each other, as in Endogens. But 



