MECHANICAL CONSTRUCTION OF PLANT-BODY 137 



and especially so in view of the fact that a perfectly elastic recovery 



after the wind-pressure ceases is a condition of its successful solution. 



A similar mechanical problem, varying with the size, presents itself 



in relation to every living plant that grows in the air. The solution 



of such problems is based upon the fact that the cells forming the plant- 



body are encysted. The necessary firmness which they show depends, 



in one way or another, upon 



the fact that a cell-wall sur- ^ ^'''- ^^• 



rounds each soft and slimy 



protoplast. 



There is reason to believe that 

 the evolution of multicellular plants 

 started from simple unicellular be- 

 ginnings. Those primitiv^e creatures, 

 the Flagellates, of which. Euglena is 

 an example, may oe regarded as 

 illustrating at the present day the 

 sort of organisms from which the 

 evolution of the higher forms may 

 have started. Euglena shows two 

 phases in its life : an active phase 

 of motion, in which the plant exists 

 as a primordial cell, that is, as 

 a protoplast without a cell-wall. 

 (Fig. 96, A, B, C.) But there is 

 also a second quiescent phase, in 

 which the protoplast is encysted, 

 that is, surrounded by a cell-wall. 

 (Fig. 97, D, E.) It is probably 

 from some such source as this that 

 the evolution of the plant-body 

 of all the higher forms originated. 

 For the encysted state of Euglena 

 corresponds structurally and me- 

 chanically to that of the cells 

 composing the ordinary tissues of 



plants ; and it has been seen that the whole plant-body, however com- 

 plicated, is built up of such encysted cells or their derivatives (Chapter II.). 

 But the primordial phase is still represented in sexual propagation. The 

 cells directly involved in that process, even in Flowering Plants, are 

 primordial cells, and in many of the lower forms they are still capable 

 of movement in water, like the active Euglena. The primordial proto- 

 plast has the advantage of free movement, but it is mechanically weak. 

 Its soft slimy consistency may suffice for small unicellular organisms ; but it 

 would be quite impossible to construct large plants from such cells without 

 some means of mechanical strengthening, especially if they are to Uve in air. 

 It is on the basis of the encysted state, strengthened and protected by cell-wall, 



Fig. 97. 



Euglena gracilis, a, Flagellate, showing in A , 

 B, C, the motile condition, without cell-wall. 

 D, is a resting cyst, with cell-wall. E, shows 

 a cyst germinating. (Highly magnified.) S. 



