SLIDING GROWTH. PROTECTION OF MERISTEMS 77 



take place unless the walls of adjacent cells become displaced rela- 

 tively to one another, or in other words, unless these walls glide or 

 slide past one another. This hypothesis necessitates the further 

 assumption that every cell which takes part in sliding growth is 

 provided with a separate cell-wall : even where the partition between 

 two adjacent cells may appear to he quite homogeneous, it must in 

 such cases actually consist of at least two distinct layers. There can 

 lie no protoplasmic connection between cells which are engaged in 

 sliding growth, since any connecting threads would inevitably be 

 ruptured in the process. The plasmodesms which are found in 

 adult tissues must therefore come into existence after sliding growth 

 has ceased. ;u 



This general discussion may conclude with a short account of the 

 arrangements which serve for the protection of meristems. Since 

 meristems are among the most delicate of vegetable tissues, they 

 evidently stand in need of effective protection against injurious 

 mechanical and climatic influences. The plant has a variety of 

 means to this end at its disposal. In the case of the outlying apical 

 meristems, it is usual for the delicate meristematic tissue to he over- 

 arched or completely enveloped by older and more resistant tissues or 

 organs. This type of protection is exceedingly widespread in its 

 occurrence, although the details of construction vary considerably 

 in different cases. It is found even among Thallophyta : in the 

 Fl'caceae, for example, the apical cell lies on the floor of a cavity 

 which communicates with the outside only by means of a narrow cleft. 

 In many Liverworts again, and also in the prothallia of Ferns, the 

 growing-point is situated at the base of a depression, which is produced 

 by the more active growth of two projecting lateral lobes. In the 

 case of the vegetative and reproductive shoots of Phanerogams 

 the task of protecting the more or less conical apical region usually 

 devolves upon the young leaves, which converge and become folded 

 together over the apex. Very often these young leaves, while serving as 

 a covering for the growing-point, nevertheless themselves also require 

 a certain amount of protection. This complication is more especially 

 characteristic of buds which have to live through the winter, or which 

 are developed underground and subsequently have to break through 

 the soil. In the case of most winter-buds all the delicate organs are 

 encased in tough leathery scale-leaves (bud-scales). In the ease of 

 subterranean structures on the other hand the entire hud often 

 assumes a drooping or reiiexed position, owing to nutation of the stem : 

 the curved region of the axis thus forms the actual boring point, 

 while the tender bud follows after in comparative safety. 



A parallel development, which likewise serves to firing the 



