THE MORPHOLOGICAL COMPOSITION OF PLANTS. 53 



tuclinal extension, which is a general characteristic of 

 Endogens and Exogens as distinguished from Acrogens, is 

 nothing more than a concomitant of their usually- vertical 

 growth;* let us now go on to consider how vertical growth 

 originates, and what are the structural changes it involves. 



§ 193. Plants depend for their prosperity mainly on air 

 and light : they dwindle where they are smothered, and 

 thrive where they can expand their leaves into free space 

 and sunshine. Those kinds which, assume prone positions, 

 consequently labour under disadvantages in being habitually 

 interfered with by one another — they are mutually shaded 

 and mutually injured. Such of them, however, as happen, 

 by variations in mode of growth, to get at all above the rest, 

 are more likely to nourish and leave offspring than the rest. 

 That is to say, natural selection will favour the more upright- 

 growing forms : individuals with structures that lift them 

 above the rest, are the fittest for the conditions ; and by the 

 continual survival of the fittest, such structures must become 

 established. There are two essentially-different ways in 

 which the integrated series of fronds above described, may 

 be modified so as to acquire the stiffness needful for main- 

 taining perpendicularity. We will consider them separately. 



A thin layer of substance gains greatly in power of re- 

 sisting a transverse strain, if it is bent round so as to form a 

 tube — witness the difference between the pliability of a sheet 

 of paper when outspread, and the rigidity of the same sheet 

 of paper when rolled up. Engineers constantly recognize 



* I am indebted to Dr Hooker for pointing out farther facts supporting this 

 view. In his Flora Antarctica, he describes the genus Zessonia (see Fig. 37) and 

 especially Z. ovata, as having a mode of growth simulating that of the Exogens. 

 The tall vertical stem thickens as it grows, by the periodical addition of layers 

 to its periphery. Among lichens, too, it seems that there is an analogous case. 

 That even Thallogcns should thus, under certain conditions, present a transversely- 

 increasing axis, shows that there is nothing absolute in the character which gives 

 the names to the two highest classes of plants, in contradistinction to the class 

 nearest to them. 



