MECHANICAL CONSTRUCTION OF PLANT-BODY 169 



of winds. In large plants this presents a serious engineering problem, 

 and especially so in view of the fact that perfect 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 primitive creatures, 

 the Flagellates, of which Euglena is 

 an example, may be 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. 104, A, B, C.) But there is 

 also a second quiescent phase, in 

 which the protoplast is encysted, fi 

 that is, surrounded by a cell-wall. 

 (Fig. 104, D, E.) It is probably 

 from some such source as this 

 that the evolution of the plant- 

 body of all the higher forms origin- 

 ated. For the encysted state of 

 Euglena corresponds structurally 

 and mechanically 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 live in air. 



Fig. 104. 



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.) (After 

 Strasburger.) 



