524 KEY. GEORGE HEKSLOW ON A THEORETICAL 



tinction from the surrounding tissue as do the cells of the epi- 

 theca proper " *. 



The significance of these differences resides in the fact that 

 the monocotyledonous type is characteristic of submerged leaves 

 of some exogenous plants. Thus, M. de Bary describes the leaf 

 of Callitriche verna as having a large pore lying over the end of 

 the vascular bundle t. " In C. uutumnalis there lie on the young 

 leaf a group of 3-8 open stomata ; in the mature leaf the guard- 

 cells of these break down, so that there remains a wide hole in 

 the epidermis/' I have examined these leaves, and others, myself, 

 and find that it is similarly the case in Myriophyllum ; and when 

 this plant is aerial in habit there is the same destruction of the 

 apical cells. It is not so, however, in Callitriche. In the sub- 

 merged and much-elongated leaf the apex is crescent-shape with 

 the concave edge outwards. This disappears in the aerial form, 

 the leaf being rounded off; intermediate stages occur in sub- 

 aquatic leaves. 



Now this more or less arrested condition of the so-called 



epithem tissue, in consequence of which the tracheides are more 



closely in contact with the pore itself, as seen in these and other 

 aquatic exogens, appears to be the permanent condition in en- 

 dogens ; and, taken with so many other features, tends to support 

 the contention that they are all descended from aquatic forms of 



exogens. 



Reproductive Organs of Endogens. — Though the influ- 

 ences of an aquatic medium powerfully affect the vegetative 

 organs of plants, the reproductive organs are not infrequently 

 considerably modified as well, especially by being degraded. 

 Flowers, however, may have an antagonistic power under certain 

 conditions, so that degeneracy due to the habitat is not universal. 

 In my work ' On the Origin of Floral Structures * I endeavoured 

 to show that conspicuousness in the floral organs was a direct 

 result of the local stimulations set up by insects i; so that if 

 they visit the flowers of aquatic plants, this stimulus may pre- 

 sumably tend to equalize, if not surpass, the degrading influ- 



* M On the Physiological Significance of Water-Glands and Nectaries," by 

 W. Gardiner ; Proc. Camb. Phil. Soc. 1883, p. 35, where the literature of the 

 subject is given. 



t Corop. Anat. of Ph. and Ferns, p. 53. 



J Conspicuousness having arisen under their influences, just as it is greatly 

 enhanced by the artificial crossing by florists. 



