Massee : Primaey Causes of Variety in Plant Structuee. Ill 



generally elevated into the air on a, stalk, has stomata ; and from this 

 point, as we ascend in the scale of vegetable development, every 

 terrestrial plant has a cuticularised covering provided with stomata. 

 The structural changes already mentioned have been the cause of a 

 corresponding specialisation of parts and division of labour, unknown 

 in the algal family, where every part of the surface served equally as 

 an organ of nutrition, so that internal arrangements for the transmis- 

 sion of liquids and gases from one part of the plant to another were 

 not required. But in land-plants the function of nutrition is restricted 

 to those aerial parts possessed of stomata, and to the roots ; and as the 

 latter alone absorb liquids, and the aerial parts gases, so the presence 

 of an additional complex structure — the fibro-vascular system, which 

 acts as a conductor of materials obtained from without, to parts of the 

 organism not capable of obtaining them directly — becomes necessary. 



The life-labour of every plant may be divided into two stages : the 

 vegetative, on which its existence as an individual depends, and the 

 reproductive, which, owiog to the limited existence of individuals, 

 provides for the continuation of the species. This provision, in the 

 simplest plants, is restricted to one kind, which may be asexual or 

 sexual ; but soon we meet- with plants possessing the two forms of 

 reproduction, and passing through the sequence of changes known as 

 alternation of generations, and it will be observed that the gradual 

 evolution of complex plants is the result of excessive development of 

 one of the two generations, the other remaining in its primitive state, 

 or even becoming rudimentary. In those algse where alternation of 

 generations is evident, the vegetative part, which is at the same time 

 the sexual generation, constitutes the bulk of the pJant, and the 

 endless variety of form and adaptation is entirely the result of 

 development in this part, which in such groups as Fucacece and 

 Floride(B attains a high degree of specialisation, the asexual spore- 

 bearing stage remaining very small and comparatively unchanged. 

 This very marked inequality of development is continued in hepatics 

 and mosses, and, as already mentioned, in Marchantia an attempt to 

 meet terrestrial conditions is seen in the presence of the epidermis ; 

 but it soon becomes evident that the highly-developed sexual genera- 

 tion common to seaweeds and mosses cannot so readily adapt itself to 

 new surroundings as the more undifferentiated and pliant asexual 

 generation, which, from the ferns upwards, constitutes the mass of 

 every plant, performs the vegetative functions, and by its marked 

 adaptability to all terrestrial conditions has enabled vegetation to 

 extend itself over the surface of the globe, and in proportion to its 



