26 BOTANY OF THE LIVING PLANT 



such as oil-globules, starch-grains, or crystals in the cytoplasm. 

 But the most marked change is the disappearance of the protoplasmic 

 body itself, so that it is not represented in the mature structure. 

 This is found to be the case in the vessels of the wood, in the cells 

 of cork, and in some other tissues (Fig. 15, B). Since the vital 

 activities reside in the protoplasm, those tissues where it is absent, 

 consisting only of cell-wall, are no longer actively living, though 

 passively they may still perform functions important in the life of 

 the plant. 



(6) Changes in the Plastids. 



These bodies are minute, and difficult to see in the young cells 

 (Fig. 9). But as the cells mature they may become more numerous 

 by division, and more prominent by their size and colour. In many 

 cells of vegetative parts they turn vivid green, and are called chloro- 

 plasts, or chlorophyll-corpuscles (Fig. 51, p. 76). They are present in 

 myriads in any green leaf, and collectively give the green colour to 

 the parts in which they occur. Other plastids may take red, or yellow 

 colours, as in petals, or in fruits, and they are called chromoplasts. 

 Others remain colourless, and are called leucoplasts, or starch- forming 

 corpuscles, because they are actively functional in the deposit of in- 

 soluble grains of starch in tubers, and elsewhere (Fig. 81, p. 124). 



By such changes as those described under the headings (1) to (6), 

 the young embryonic cells may be transformed into the various 

 tissues that make up the mature parts. Originally the cells were all 

 alike ; as they become mature they are liable to be differentiated 

 and specialised for different functions. It may be held as probable 

 that what is seen in the individual development is a reasonable guide 

 to what actually took place in the evolution of the race. It is prob- 

 able that plants with little or no differentiation of tissues, that is 

 Cellular Plants such as the Algae, preceded in the history of Evolution 

 the more complex Vascular Plants. These with their higher state 

 of tissue-differentiation constitute the leading feature of the Flora 

 of exposed Land-Surfaces. 



Continuity of Protoplasm. 



The cells of living tissues all share in a common physiological life, 

 and are in intimate relation to one another. But cell-walls separating 

 adjoining cells do not form complete barriers between their protoplasts. 

 In most mature tissues a Continuity of Protoplasm may be demon- 

 strated, from cell to cell. It is established by means of fine connecting 



