The Plant Kingdom - 609 



Fig. 31-16. Wheat rust, growing on 

 wheat stubble (left) and on barberry 

 leaves (right). This basidiomycete 

 (Pucc/nia graminis) is a very destruc- 

 tive parasite. Summer spores of this 

 rust can pass from wheat plant to 

 wheat plant. But the fall spores, 

 which grow upon wheat stubble, can 

 continue to live only if they manage 

 to infest the leaves of the common 

 barberry bush. Only winter spores, 

 formed in the barberry leaves, are 

 capable of surviving through a cold 

 season. Consequently, it is the winter 

 spores (at least in northern regions) 

 that reinfect the wheat crop when 

 the following spring arrives. The 

 name "rust" derives from the orange 

 color of the summer sporangia, 

 formed on the leaves of the wheat. 

 (Photos by Benjamin Koehler; from 

 The Plant World.) 



soil and helped to prepare the land envi- 

 ronment for the coming of other plants. 



SUBKINGDOM EMBRYOPHYTA 



Essentially the various groups of Thallo- 

 phyta, which have been described in the 

 foregoing sections, are relatively simple 

 water-dwelling plants, despite the fact that a 

 few species show some slight degree of 

 adaptation to life upon the land. The Thal- 

 lophyta appear to have originated quite early 

 in the Proterozoic era (Fig. 29-10), at which 

 time no terrestrial plants existed. Land 

 plants, in fact, did not begin to appear until 

 well after the early days of the Paleozoic era. 

 At this point, therefore, the question of how 

 plants became adapted to terrestrial condi- 

 tions becomes pertinent. 



Adaptation of Plants to the Land Environment. 

 Land-adapted nutritional organs, such as 

 the leaves, roots, and stems of higher plants, 

 did not spring suddenly full-fledged into 

 being; and the same is true of the specialized 

 tissues such as the conducting units of the 



xylem and phloem, and the well-developed 

 water-proofed epidermal tissues, which are 

 so essential to well-adapted land species 

 (Chap. 13). It is interesting to note, there- 

 fore, how these nutritional adaptations did 

 arise gradually in the various groups of 

 higher plants. 



The land environment also imposes two 

 serious problems upon the reproductive 

 mechanisms of the plant. If the plant is not 

 submerged in water: (1) How can the sperm 

 manage to swim to the eggs? and (2) How 

 can the unicellular zygote manage to survive 

 while the new plant is developing its first 

 roots, stem, and leaves? No truly successful 

 land species could appear, in fact, until the 

 reproductive mechanisms became adjusted 

 to meet these basic problems. 



The first problem did not reach a truly 

 satisfactory solution in any of the earlier 

 groups. Consequently the more primitive 

 land species are restricted, more or less com- 

 pletely, to areas where, at least periodically, 

 there is an abundance of water in which the 

 sperm can swin. But the highest plant 



